Matthias, Barry, and I looked at this at pycon today.  It looks a bit like the original intent was to have
SO = ".so"
SOABI = "cpython-32mu"

and then CPython extension module suffixes would be:

if SOABI:
    so_ext = ''.join(".", SOABI, SO)
else:
    so_ext = SO

This would need to be used in distutils/commands/build_ext.py  We weren't sure if there are other places in the code that would need it as well but a quick build of a module which uses libraries and needs C extensions showed that this seems to work.

The one worrisome question is whether more people have come to rely on the SO variable holding the extension module suffix or if more code was broken by the extension module suffix replacing the library suffix in the SO variable.  Answering that might better show us whether to change these variables back to their original meanings or to create brand new variables that have the correct values.

We also discovered the reason the current version appears to work with python-pillow on Ubuntu boxes but not Fedora.  The find_library_files() code first checks for library names that would match with the "shared" library suffix.  If that fails, it falls back to looking for the "static" library suffix.  On Fedora, there are no static libraries so this function just fails to find the library.  On Ubuntu, the code finds the static libraries and returns that.  This causes the code in python-pillow to attempt to link to the library with -ljpeg, -lpng, etc...  Since the shared libraries actually are present, the compiler and linker use the shared versions even though python only found the static versions.