Fix up libpthread linking error with dtls/server-dtls-threaded

Signed-off-by: Liu Qun <517067180@qq.com>

dtls/Makefile

@@ -28,6 +28,10 @@ all: $(TARGETS)
 debug: CFLAGS+=$(DEBUG_FLAGS)
 debug: all
 
+# add the -pthread flag and -lpthread lib to any threaded examples
+%-threaded: CFLAGS+=-pthread
+%-threaded: LIBS+=-lpthread
+
 # build template
 %: %.c
 	$(CC) -o $@ $< $(CFLAGS) $(LIBS)