Windows had an issue where it was trying to build the CFFI module after
it had already imported the CFFI module. Which caused permissions
errors during builds.
This fix does several things to make the Windows build work properly and
improve the Linux build too:
* The CFFI module is only build when needed, not as part of an sdist
package
* Version numbering spilt out into separate file so __init__.py import
is not required
* Merged _build_ffi.py and _build_wolfssl.py into one file
* Made CFFI only build when called as an executable (which happens
during binary build time)
* Make tox use bdist-wheel instead of sdist
Lealem Amedie
Daniele Lacamera
Andrew Hutchings