Fixed #3386: the optional prefix argument was ignored under OS2 and NT in distutils.sysconfig.get_python_lib

#3986 replacing string and types call (like in the Py3k branch), and put exec_msg call at the right place

the only user of this is Lib/distutils/tests/test_build_ext.py (in order
to find the source for xxmodule.c).  I'm not sure if other platforms
need similar tweaks, I'm not brave enough to attempt it without being
able to test.

Since "srcdir" should now be defined on all platforms, use it to
find the module source.

directory other than the source directory, simplify the test code in
test_sysconfig.py.

the source.  Also, define 'srcdir' on non-posix platforms.

Reviewed by Amaury Forgeot d'Arc.