diff --git a/configure.ac b/configure.ac
index 693d2a42..44d871c2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10,6 +10,7 @@ AC_CONFIG_AUX_DIR([build-aux])
 : ${CFLAGS=""}
 
 AC_CANONICAL_HOST
+AC_CANONICAL_TARGET
 
 AM_INIT_AUTOMAKE([1.11 -Wall -Werror -Wno-portability foreign tar-ustar subdir-objects no-define color-tests])
 
@@ -45,8 +46,7 @@ AC_PROG_CC
 AM_PROG_CC_C_O
 AC_PROG_INSTALL
 
-# Checks for header files.
-AC_HEADER_STDC
+# Checks for type sizes.
 AC_CHECK_SIZEOF([long long])
 AC_CHECK_SIZEOF([long])
 AC_CHECK_SIZEOF([off_t])
diff --git a/m4/ax_am_jobserver.m4 b/m4/ax_am_jobserver.m4
index dfbcdbb2..7da8d5e7 100644
--- a/m4/ax_am_jobserver.m4
+++ b/m4/ax_am_jobserver.m4
@@ -33,7 +33,9 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 8
+#serial 8.1
+# Local update: Make the increment of enable-jobserver so it isn't executable
+# and is more posix syntax.
 
 AC_DEFUN([AX_AM_JOBSERVER], [
     AC_REQUIRE([AX_COUNT_CPUS])
@@ -45,7 +47,7 @@ AC_DEFUN([AX_AM_JOBSERVER], [
     ],, [enable_jobserver=m4_ifval([$1],[$1],[yes])])
     if test "x$enable_jobserver" = "xyes"; then
         enable_jobserver=$CPU_COUNT
-        ((enable_jobserver++))
+        : $((enable_jobserver+=1))
     fi
     m4_pattern_allow(AM_MAKEFLAGS)
     if test "x$enable_jobserver" != "xno"; then
diff --git a/m4/ax_create_generic_config.m4 b/m4/ax_create_generic_config.m4
index 535838f4..59aeee61 100644
--- a/m4/ax_create_generic_config.m4
+++ b/m4/ax_create_generic_config.m4
@@ -9,7 +9,7 @@
 # DESCRIPTION
 #
 #   Creates a generic PACKAGE-config file that has all the things that you
-#   want, hmm, well, atleast it has --cflags, --version, --libs. Ahhm, did
+#   want, hmm, well, at least it has --cflags, --version, --libs. Ahhm, did
 #   you see ax_path_generic in the autoconf-archive? ;-)
 #
 #   this macros saves you all the typing for a pkg-config.in script, you
@@ -78,7 +78,7 @@ echo 'package="'$P'"' >>$F
 echo 'version="'$V'"' >>$F
 echo 'libs="'$L'"' >>$F
 echo ' ' >>$F
-# in the order of occurence a standard automake Makefile
+# in the order of occurrence a standard automake Makefile
 echo 'prefix="'$prefix'"' >>$F
 echo 'exec_prefix="'$exec_prefix'"' >>$F
 echo 'bindir="'$bindir'"' >>$F
diff --git a/m4/ax_harden_compiler_flags.m4 b/m4/ax_harden_compiler_flags.m4
index a2067f07..90885562 100644
--- a/m4/ax_harden_compiler_flags.m4
+++ b/m4/ax_harden_compiler_flags.m4
@@ -60,8 +60,11 @@
 # AX_APPEND_COMPILE_FLAGS([-Wlogical-op],,[$ax_append_compile_cflags_extra])
 # AX_APPEND_COMPILE_FLAGS([-fstack-check],,[$ax_append_compile_cflags_extra]) -- problems with fastmath stack size checks
 # AX_APPEND_COMPILE_FLAGS([-floop-parallelize-all],,[$ax_append_compile_cflags_extra]) -- causes RSA verify problem on x64
+# AX_APPEND_COMPILE_FLAGS([-Wunreachable-code],,[$ax_append_compile_cflags_extra])  -- older clang and when gcc had it are buggy
+# AX_APPEND_COMPILE_FLAGS([-fPIE],,[$ax_append_compile_cflags_extra]) -- Flag for executables not libraries
 
-#serial 4
+#serial 4.2
+# changes: deleted the clearing of CFLAGS
 
   AC_DEFUN([AX_HARDEN_LINKER_FLAGS], [
       AX_REQUIRE_DEFINED([AX_CHECK_LINK_FLAG])
@@ -100,63 +103,66 @@
 
       ac_cv_warnings_as_errors=no
       ax_append_compile_cflags_extra=
-      AX_APPEND_COMPILE_FLAGS([-Werror],[ax_append_compile_cflags_extra])
-      AS_IF([test "x$ac_cv_vcs_checkout" = "xyes"],
-            [AX_APPEND_COMPILE_FLAGS([-Werror],[AM_CFLAGS])
-             ac_cv_warnings_as_errors="yes"])
+      AS_IF([test "$ac_cv_vcs_checkout" = "yes"],[
+        AX_APPEND_COMPILE_FLAGS([-Werror])
+        ac_cv_warnings_as_errors=yes
+        ],[
+        AX_APPEND_COMPILE_FLAGS([-Werror],[ax_append_compile_cflags_extra])
+        ])
 
-dnl      The main configure script handles setting the debugging flags.
-dnl      AS_IF([test "$ax_enable_debug" = "yes"], [
-dnl        AX_APPEND_COMPILE_FLAGS([-g],[AM_CFLAGS])
-dnl        AX_APPEND_COMPILE_FLAGS([-ggdb],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-dnl        AX_APPEND_COMPILE_FLAGS([-O0],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-dnl        ],[])
+      AS_IF([test "$ax_enable_debug" = "yes"], [
+        AX_APPEND_COMPILE_FLAGS([-g])
+        AX_APPEND_COMPILE_FLAGS([-ggdb],,[$ax_append_compile_cflags_extra])
+        AX_APPEND_COMPILE_FLAGS([-O0],,[$ax_append_compile_cflags_extra])
+        ],[])
 
-      AX_APPEND_COMPILE_FLAGS([-Wno-pragmas],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wno-pragmas],,[$ax_append_compile_cflags_extra])
 
-      AX_APPEND_COMPILE_FLAGS([-Wall],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wno-strict-aliasing],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wextra],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wunknown-pragmas],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wthis-test-should-fail],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wall],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wno-strict-aliasing],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wextra],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wunknown-pragmas],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wthis-test-should-fail],,[$ax_append_compile_cflags_extra])
       dnl Anything below this comment please keep sorted.
-      AS_IF([test "x$CC" != "xclang"],
-            [AX_APPEND_COMPILE_FLAGS([--param=ssp-buffer-size=1],[AM_CFLAGS],[$ax_append_compile_cflags_extra])])
-      AX_APPEND_COMPILE_FLAGS([-Waddress],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Warray-bounds],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wbad-function-cast],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
+      AS_IF([test "$CC" = "clang"],[],[
+        AX_APPEND_COMPILE_FLAGS([--param=ssp-buffer-size=1],,[$ax_append_compile_cflags_extra])
+        ])
+      AX_APPEND_COMPILE_FLAGS([-Waddress],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Warray-bounds],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wbad-function-cast],,[$ax_append_compile_cflags_extra])
       dnl Not in use -Wc++-compat
-      AX_APPEND_COMPILE_FLAGS([-Wchar-subscripts],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wcomment],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wfloat-equal],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wformat-security],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wformat=2],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wmaybe-uninitialized],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wmissing-field-initializers],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wmissing-noreturn],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wmissing-prototypes],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wnested-externs],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wnormalized=id],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Woverride-init],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wpointer-arith],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wpointer-sign],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wredundant-decls],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wshadow],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wshorten-64-to-32],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wsign-compare],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wstrict-overflow=1],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wstrict-prototypes],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wswitch-enum],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wundef],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wunused],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wunused-result],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wunused-variable],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-Wwrite-strings],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
-      AX_APPEND_COMPILE_FLAGS([-fwrapv],[AM_CFLAGS],[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wchar-subscripts],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wcomment],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wfloat-equal],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wformat-security],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wformat=2],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wmaybe-uninitialized],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wmissing-field-initializers],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wmissing-noreturn],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wmissing-prototypes],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wnested-externs],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wnormalized=id],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Woverride-init],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wpointer-arith],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wpointer-sign],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wredundant-decls],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wshadow],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wshorten-64-to-32],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wsign-compare],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wstrict-overflow=1],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wstrict-prototypes],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wswitch-enum],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wundef],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wunused],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wunused-result],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wunused-variable],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-Wwrite-strings],,[$ax_append_compile_cflags_extra])
+      AX_APPEND_COMPILE_FLAGS([-fwrapv],,[$ax_append_compile_cflags_extra])
       AC_LANG_POP
       ])
 
   AC_DEFUN([AX_HARDEN_CXX_COMPILER_FLAGS], [
+      AC_REQUIRE_DEFINED([AX_APPEND_COMPILE_FLAGS])
       AC_REQUIRE([AX_HARDEN_CC_COMPILER_FLAGS])
       AC_LANG_PUSH([C++])
 
diff --git a/m4/ax_pthread.m4 b/m4/ax_pthread.m4
index ada7071f..e5858e50 100644
--- a/m4/ax_pthread.m4
+++ b/m4/ax_pthread.m4
@@ -14,20 +14,24 @@
 #   flags that are needed. (The user can also force certain compiler
 #   flags/libs to be tested by setting these environment variables.)
 #
-#   Also sets PTHREAD_CC to any special C compiler that is needed for
-#   multi-threaded programs (defaults to the value of CC otherwise). (This
-#   is necessary on AIX to use the special cc_r compiler alias.)
+#   Also sets PTHREAD_CC and PTHREAD_CXX to any special C compiler that is
+#   needed for multi-threaded programs (defaults to the value of CC
+#   respectively CXX otherwise). (This is necessary on e.g. AIX to use the
+#   special cc_r/CC_r compiler alias.)
 #
 #   NOTE: You are assumed to not only compile your program with these flags,
 #   but also to link with them as well. For example, you might link with
 #   $PTHREAD_CC $CFLAGS $PTHREAD_CFLAGS $LDFLAGS ... $PTHREAD_LIBS $LIBS
+#   $PTHREAD_CXX $CXXFLAGS $PTHREAD_CFLAGS $LDFLAGS ... $PTHREAD_LIBS $LIBS
 #
 #   If you are only building threaded programs, you may wish to use these
 #   variables in your default LIBS, CFLAGS, and CC:
 #
 #     LIBS="$PTHREAD_LIBS $LIBS"
 #     CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+#     CXXFLAGS="$CXXFLAGS $PTHREAD_CFLAGS"
 #     CC="$PTHREAD_CC"
+#     CXX="$PTHREAD_CXX"
 #
 #   In addition, if the PTHREAD_CREATE_JOINABLE thread-attribute constant
 #   has a nonstandard name, this macro defines PTHREAD_CREATE_JOINABLE to
@@ -55,6 +59,7 @@
 #
 #   Copyright (c) 2008 Steven G. Johnson <stevenj@alum.mit.edu>
 #   Copyright (c) 2011 Daniel Richard G. <skunk@iSKUNK.ORG>
+#   Copyright (c) 2019 Marc Stevens <marc.stevens@cwi.nl>
 #
 #   This program is free software: you can redistribute it and/or modify it
 #   under the terms of the GNU General Public License as published by the
@@ -82,11 +87,11 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 24
+#serial 30
 
 AU_ALIAS([ACX_PTHREAD], [AX_PTHREAD])
 AC_DEFUN([AX_PTHREAD], [
-AC_REQUIRE([AC_CANONICAL_HOST])
+AC_REQUIRE([AC_CANONICAL_TARGET])
 AC_REQUIRE([AC_PROG_CC])
 AC_REQUIRE([AC_PROG_SED])
 AC_LANG_PUSH([C])
@@ -104,6 +109,7 @@ if test "x$PTHREAD_CFLAGS$PTHREAD_LIBS" != "x"; then
         ax_pthread_save_CFLAGS="$CFLAGS"
         ax_pthread_save_LIBS="$LIBS"
         AS_IF([test "x$PTHREAD_CC" != "x"], [CC="$PTHREAD_CC"])
+        AS_IF([test "x$PTHREAD_CXX" != "x"], [CXX="$PTHREAD_CXX"])
         CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
         LIBS="$PTHREAD_LIBS $LIBS"
         AC_MSG_CHECKING([for pthread_join using $CC $PTHREAD_CFLAGS $PTHREAD_LIBS])
@@ -123,10 +129,12 @@ fi
 # (e.g. DEC) have both -lpthread and -lpthreads, where one of the
 # libraries is broken (non-POSIX).
 
-# Create a list of thread flags to try.  Items starting with a "-" are
-# C compiler flags, and other items are library names, except for "none"
-# which indicates that we try without any flags at all, and "pthread-config"
-# which is a program returning the flags for the Pth emulation library.
+# Create a list of thread flags to try. Items with a "," contain both
+# C compiler flags (before ",") and linker flags (after ","). Other items
+# starting with a "-" are C compiler flags, and remaining items are
+# library names, except for "none" which indicates that we try without
+# any flags at all, and "pthread-config" which is a program returning
+# the flags for the Pth emulation library.
 
 ax_pthread_flags="pthreads none -Kthread -pthread -pthreads -mthreads pthread --thread-safe -mt pthread-config"
 
@@ -150,7 +158,7 @@ ax_pthread_flags="pthreads none -Kthread -pthread -pthreads -mthreads pthread --
 # --thread-safe: KAI C++
 # pthread-config: use pthread-config program (for GNU Pth library)
 
-case $host_os in
+case $target_os in
 
         freebsd*)
 
@@ -194,36 +202,10 @@ case $host_os in
         # that too in a future libc.)  So we'll check first for the
         # standard Solaris way of linking pthreads (-mt -lpthread).
 
-        ax_pthread_flags="-mt,pthread pthread $ax_pthread_flags"
+        ax_pthread_flags="-mt,-lpthread pthread $ax_pthread_flags"
         ;;
 esac
 
-# GCC generally uses -pthread, or -pthreads on some platforms (e.g. SPARC)
-
-AS_IF([test "x$GCC" = "xyes"],
-      [ax_pthread_flags="-pthread -pthreads $ax_pthread_flags"])
-
-# The presence of a feature test macro requesting re-entrant function
-# definitions is, on some systems, a strong hint that pthreads support is
-# correctly enabled
-
-case $host_os in
-        darwin* | hpux* | linux* | osf* | solaris*)
-        ax_pthread_check_macro="_REENTRANT"
-        ;;
-
-        aix*)
-        ax_pthread_check_macro="_THREAD_SAFE"
-        ;;
-
-        *)
-        ax_pthread_check_macro="--"
-        ;;
-esac
-AS_IF([test "x$ax_pthread_check_macro" = "x--"],
-      [ax_pthread_check_cond=0],
-      [ax_pthread_check_cond="!defined($ax_pthread_check_macro)"])
-
 # Are we compiling with Clang?
 
 AC_CACHE_CHECK([whether $CC is Clang],
@@ -242,83 +224,47 @@ AC_CACHE_CHECK([whether $CC is Clang],
     ])
 ax_pthread_clang="$ax_cv_PTHREAD_CLANG"
 
-ax_pthread_clang_warning=no
 
-# Clang needs special handling, because older versions handle the -pthread
-# option in a rather... idiosyncratic way
+# GCC generally uses -pthread, or -pthreads on some platforms (e.g. SPARC)
 
-if test "x$ax_pthread_clang" = "xyes"; then
+# Note that for GCC and Clang -pthread generally implies -lpthread,
+# except when -nostdlib is passed.
+# This is problematic using libtool to build C++ shared libraries with pthread:
+# [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25460
+# [2] https://bugzilla.redhat.com/show_bug.cgi?id=661333
+# [3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=468555
+# To solve this, first try -pthread together with -lpthread for GCC
 
-        # Clang takes -pthread; it has never supported any other flag
+AS_IF([test "x$GCC" = "xyes"],
+      [ax_pthread_flags="-pthread,-lpthread -pthread -pthreads $ax_pthread_flags"])
 
-        # (Note 1: This will need to be revisited if a system that Clang
-        # supports has POSIX threads in a separate library.  This tends not
-        # to be the way of modern systems, but it's conceivable.)
+# Clang takes -pthread (never supported any other flag), but we'll try with -lpthread first
 
-        # (Note 2: On some systems, notably Darwin, -pthread is not needed
-        # to get POSIX threads support; the API is always present and
-        # active.  We could reasonably leave PTHREAD_CFLAGS empty.  But
-        # -pthread does define _REENTRANT, and while the Darwin headers
-        # ignore this macro, third-party headers might not.)
+AS_IF([test "x$ax_pthread_clang" = "xyes"],
+      [ax_pthread_flags="-pthread,-lpthread -pthread"])
 
-        PTHREAD_CFLAGS="-pthread"
-        PTHREAD_LIBS=
 
-        ax_pthread_ok=yes
+# The presence of a feature test macro requesting re-entrant function
+# definitions is, on some systems, a strong hint that pthreads support is
+# correctly enabled
 
-        # However, older versions of Clang make a point of warning the user
-        # that, in an invocation where only linking and no compilation is
-        # taking place, the -pthread option has no effect ("argument unused
-        # during compilation").  They expect -pthread to be passed in only
-        # when source code is being compiled.
-        #
-        # Problem is, this is at odds with the way Automake and most other
-        # C build frameworks function, which is that the same flags used in
-        # compilation (CFLAGS) are also used in linking.  Many systems
-        # supported by AX_PTHREAD require exactly this for POSIX threads
-        # support, and in fact it is often not straightforward to specify a
-        # flag that is used only in the compilation phase and not in
-        # linking.  Such a scenario is extremely rare in practice.
-        #
-        # Even though use of the -pthread flag in linking would only print
-        # a warning, this can be a nuisance for well-run software projects
-        # that build with -Werror.  So if the active version of Clang has
-        # this misfeature, we search for an option to squash it.
+case $target_os in
+        darwin* | hpux* | linux* | osf* | solaris*)
+        ax_pthread_check_macro="_REENTRANT"
+        ;;
 
-        AC_CACHE_CHECK([whether Clang needs flag to prevent "argument unused" warning when linking with -pthread],
-            [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG],
-            [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG=unknown
-             # Create an alternate version of $ac_link that compiles and
-             # links in two steps (.c -> .o, .o -> exe) instead of one
-             # (.c -> exe), because the warning occurs only in the second
-             # step
-             ax_pthread_save_ac_link="$ac_link"
-             ax_pthread_sed='s/conftest\.\$ac_ext/conftest.$ac_objext/g'
-             ax_pthread_link_step=`$as_echo "$ac_link" | sed "$ax_pthread_sed"`
-             ax_pthread_2step_ac_link="($ac_compile) && (echo ==== >&5) && ($ax_pthread_link_step)"
-             ax_pthread_save_CFLAGS="$CFLAGS"
-             for ax_pthread_try in '' -Qunused-arguments -Wno-unused-command-line-argument unknown; do
-                AS_IF([test "x$ax_pthread_try" = "xunknown"], [break])
-                CFLAGS="-Werror -Wunknown-warning-option $ax_pthread_try -pthread $ax_pthread_save_CFLAGS"
-                ac_link="$ax_pthread_save_ac_link"
-                AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
-                    [ac_link="$ax_pthread_2step_ac_link"
-                     AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
-                         [break])
-                    ])
-             done
-             ac_link="$ax_pthread_save_ac_link"
-             CFLAGS="$ax_pthread_save_CFLAGS"
-             AS_IF([test "x$ax_pthread_try" = "x"], [ax_pthread_try=no])
-             ax_cv_PTHREAD_CLANG_NO_WARN_FLAG="$ax_pthread_try"
-            ])
+        aix*)
+        ax_pthread_check_macro="_THREAD_SAFE"
+        ;;
 
-        case "$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG" in
-                no | unknown) ;;
-                *) PTHREAD_CFLAGS="$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG $PTHREAD_CFLAGS" ;;
-        esac
+        *)
+        ax_pthread_check_macro="--"
+        ;;
+esac
+AS_IF([test "x$ax_pthread_check_macro" = "x--"],
+      [ax_pthread_check_cond=0],
+      [ax_pthread_check_cond="!defined($ax_pthread_check_macro)"])
 
-fi # $ax_pthread_clang = yes
 
 if test "x$ax_pthread_ok" = "xno"; then
 for ax_pthread_try_flag in $ax_pthread_flags; do
@@ -328,10 +274,10 @@ for ax_pthread_try_flag in $ax_pthread_flags; do
                 AC_MSG_CHECKING([whether pthreads work without any flags])
                 ;;
 
-                -mt,pthread)
-                AC_MSG_CHECKING([whether pthreads work with -mt -lpthread])
-                PTHREAD_CFLAGS="-mt"
-                PTHREAD_LIBS="-lpthread"
+                *,*)
+                PTHREAD_CFLAGS=`echo $ax_pthread_try_flag | sed "s/^\(.*\),\(.*\)$/\1/"`
+                PTHREAD_LIBS=`echo $ax_pthread_try_flag | sed "s/^\(.*\),\(.*\)$/\2/"`
+                AC_MSG_CHECKING([whether pthreads work with "$PTHREAD_CFLAGS" and "$PTHREAD_LIBS"])
                 ;;
 
                 -*)
@@ -368,10 +314,16 @@ for ax_pthread_try_flag in $ax_pthread_flags; do
         # We try pthread_create on general principles.
 
         AC_LINK_IFELSE([AC_LANG_PROGRAM([#include <pthread.h>
-dnl#                       if $ax_pthread_check_cond
-dnl#                        error "$ax_pthread_check_macro must be defined"
-dnl#                       endif
-                        static void routine(void *a) { a = 0; }
+#                       if $ax_pthread_check_cond
+#                        error "$ax_pthread_check_macro must be defined"
+#                       endif
+                        static void *some_global = NULL;
+                        static void routine(void *a)
+                          {
+                             /* To avoid any unused-parameter or
+                                unused-but-set-parameter warning.  */
+                             some_global = a;
+                          }
                         static void *start_routine(void *a) { return a; }],
                        [pthread_t th; pthread_attr_t attr;
                         pthread_create(&th, 0, start_routine, 0);
@@ -393,6 +345,80 @@ dnl#                       endif
 done
 fi
 
+
+# Clang needs special handling, because older versions handle the -pthread
+# option in a rather... idiosyncratic way
+
+if test "x$ax_pthread_clang" = "xyes"; then
+
+        # Clang takes -pthread; it has never supported any other flag
+
+        # (Note 1: This will need to be revisited if a system that Clang
+        # supports has POSIX threads in a separate library.  This tends not
+        # to be the way of modern systems, but it's conceivable.)
+
+        # (Note 2: On some systems, notably Darwin, -pthread is not needed
+        # to get POSIX threads support; the API is always present and
+        # active.  We could reasonably leave PTHREAD_CFLAGS empty.  But
+        # -pthread does define _REENTRANT, and while the Darwin headers
+        # ignore this macro, third-party headers might not.)
+
+        # However, older versions of Clang make a point of warning the user
+        # that, in an invocation where only linking and no compilation is
+        # taking place, the -pthread option has no effect ("argument unused
+        # during compilation").  They expect -pthread to be passed in only
+        # when source code is being compiled.
+        #
+        # Problem is, this is at odds with the way Automake and most other
+        # C build frameworks function, which is that the same flags used in
+        # compilation (CFLAGS) are also used in linking.  Many systems
+        # supported by AX_PTHREAD require exactly this for POSIX threads
+        # support, and in fact it is often not straightforward to specify a
+        # flag that is used only in the compilation phase and not in
+        # linking.  Such a scenario is extremely rare in practice.
+        #
+        # Even though use of the -pthread flag in linking would only print
+        # a warning, this can be a nuisance for well-run software projects
+        # that build with -Werror.  So if the active version of Clang has
+        # this misfeature, we search for an option to squash it.
+
+        AC_CACHE_CHECK([whether Clang needs flag to prevent "argument unused" warning when linking with -pthread],
+            [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG],
+            [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG=unknown
+             # Create an alternate version of $ac_link that compiles and
+             # links in two steps (.c -> .o, .o -> exe) instead of one
+             # (.c -> exe), because the warning occurs only in the second
+             # step
+             ax_pthread_save_ac_link="$ac_link"
+             ax_pthread_sed='s/conftest\.\$ac_ext/conftest.$ac_objext/g'
+             ax_pthread_link_step=`AS_ECHO(["$ac_link"]) | sed "$ax_pthread_sed"`
+             ax_pthread_2step_ac_link="($ac_compile) && (echo ==== >&5) && ($ax_pthread_link_step)"
+             ax_pthread_save_CFLAGS="$CFLAGS"
+             for ax_pthread_try in '' -Qunused-arguments -Wno-unused-command-line-argument unknown; do
+                AS_IF([test "x$ax_pthread_try" = "xunknown"], [break])
+                CFLAGS="-Werror -Wunknown-warning-option $ax_pthread_try -pthread $ax_pthread_save_CFLAGS"
+                ac_link="$ax_pthread_save_ac_link"
+                AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
+                    [ac_link="$ax_pthread_2step_ac_link"
+                     AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
+                         [break])
+                    ])
+             done
+             ac_link="$ax_pthread_save_ac_link"
+             CFLAGS="$ax_pthread_save_CFLAGS"
+             AS_IF([test "x$ax_pthread_try" = "x"], [ax_pthread_try=no])
+             ax_cv_PTHREAD_CLANG_NO_WARN_FLAG="$ax_pthread_try"
+            ])
+
+        case "$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG" in
+                no | unknown) ;;
+                *) PTHREAD_CFLAGS="$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG $PTHREAD_CFLAGS" ;;
+        esac
+
+fi # $ax_pthread_clang = yes
+
+
+
 # Various other checks:
 if test "x$ax_pthread_ok" = "xyes"; then
         ax_pthread_save_CFLAGS="$CFLAGS"
@@ -424,7 +450,7 @@ if test "x$ax_pthread_ok" = "xyes"; then
         AC_CACHE_CHECK([whether more special flags are required for pthreads],
             [ax_cv_PTHREAD_SPECIAL_FLAGS],
             [ax_cv_PTHREAD_SPECIAL_FLAGS=no
-             case $host_os in
+             case $target_os in
              solaris*)
              ax_cv_PTHREAD_SPECIAL_FLAGS="-D_POSIX_PTHREAD_SEMANTICS"
              ;;
@@ -438,7 +464,8 @@ if test "x$ax_pthread_ok" = "xyes"; then
         AC_CACHE_CHECK([for PTHREAD_PRIO_INHERIT],
             [ax_cv_PTHREAD_PRIO_INHERIT],
             [AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <pthread.h>]],
-                                             [[int i = PTHREAD_PRIO_INHERIT;]])],
+                                             [[int i = PTHREAD_PRIO_INHERIT;
+                                               return i;]])],
                             [ax_cv_PTHREAD_PRIO_INHERIT=yes],
                             [ax_cv_PTHREAD_PRIO_INHERIT=no])
             ])
@@ -453,25 +480,35 @@ if test "x$ax_pthread_ok" = "xyes"; then
 
         # More AIX lossage: compile with *_r variant
         if test "x$GCC" != "xyes"; then
-            case $host_os in
+            case $target_os in
                 aix*)
                 AS_CASE(["x/$CC"],
                     [x*/c89|x*/c89_128|x*/c99|x*/c99_128|x*/cc|x*/cc128|x*/xlc|x*/xlc_v6|x*/xlc128|x*/xlc128_v6],
                     [#handle absolute path differently from PATH based program lookup
                      AS_CASE(["x$CC"],
                          [x/*],
-                         [AS_IF([AS_EXECUTABLE_P([${CC}_r])],[PTHREAD_CC="${CC}_r"])],
-                         [AC_CHECK_PROGS([PTHREAD_CC],[${CC}_r],[$CC])])])
+                         [
+			   AS_IF([AS_EXECUTABLE_P([${CC}_r])],[PTHREAD_CC="${CC}_r"])
+			   AS_IF([test "x${CXX}" != "x"], [AS_IF([AS_EXECUTABLE_P([${CXX}_r])],[PTHREAD_CXX="${CXX}_r"])])
+			 ],
+                         [
+			   AC_CHECK_PROGS([PTHREAD_CC],[${CC}_r],[$CC])
+			   AS_IF([test "x${CXX}" != "x"], [AC_CHECK_PROGS([PTHREAD_CXX],[${CXX}_r],[$CXX])])
+			 ]
+                     )
+                    ])
                 ;;
             esac
         fi
 fi
 
 test -n "$PTHREAD_CC" || PTHREAD_CC="$CC"
+test -n "$PTHREAD_CXX" || PTHREAD_CXX="$CXX"
 
 AC_SUBST([PTHREAD_LIBS])
 AC_SUBST([PTHREAD_CFLAGS])
 AC_SUBST([PTHREAD_CC])
+AC_SUBST([PTHREAD_CXX])
 
 # Finally, execute ACTION-IF-FOUND/ACTION-IF-NOT-FOUND:
 if test "x$ax_pthread_ok" = "xyes"; then
diff --git a/m4/ax_vcs_checkout.m4 b/m4/ax_vcs_checkout.m4
index 8047b65e..4636b58e 100644
--- a/m4/ax_vcs_checkout.m4
+++ b/m4/ax_vcs_checkout.m4
@@ -45,7 +45,10 @@
 #  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 #  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#serial 6
+#serial 6.1
+#
+# Added tweak for git. The base repo's .git is a directory. Any worktree's
+# .git is a file. Use -e to check for either dir or file.
 
 AC_DEFUN([AX_VCS_SYSTEM],
     [AC_PREREQ([2.63])dnl
@@ -54,7 +57,7 @@ AC_DEFUN([AX_VCS_SYSTEM],
       AS_IF([test -d ".bzr"],[ac_cv_vcs_system="bazaar"])
       AS_IF([test -d ".svn"],[ac_cv_vcs_system="svn"])
       AS_IF([test -d ".hg"],[ac_cv_vcs_system="mercurial"])
-      AS_IF([test -d ".git"],[ac_cv_vcs_system="git"])
+      AS_IF([test -e ".git"],[ac_cv_vcs_system="git"])
       ])
     AC_DEFINE_UNQUOTED([VCS_SYSTEM],["$ac_cv_vcs_system"],[VCS system])
     ])