This patch adds support for the GCC visibility attributes to the PyAPI_* macros (currently only used for Windows.) GCC's default visibility is 'public', but can be changed to 'hidden' with the '-fvisibility=hidden' argument; see http://gcc.gnu.org/wiki/Visibility. This patch does not make the build use that, it merely makes Python function correctly when the default visibility *is* changed. (The benefit of building Python with -fvisibility=hidden is very small, as it causes only a handful of symbols to not be exported. When embedding Python, though, this can make a lot of difference.)

The patch also fixes a few modules that don't use PyMODINIT_FUNC for their module-init function definitions, like they should.