Fix GCC on Darwin for Python 3.

distutils.sysconfig.get_config_var('CC') returns the compiler used to
build Python, which may be not installed on the system where uWSGI is
being built.

In my case, I grabbed Python 3.3.2 from python.org: it was apparently
compiled with 'gcc-4.2', but my system only has 'gcc' (symlink) and
'llvm-gcc-4.2'.  This change tries to detect this situation (only on
Darwin) and sets GCC to 'llvm-' + compiler returned by sysconfig.
Merge pull request #317 from anaconda/fix-gcc-darwin-llvm

Fix GCC on Darwin for Python 3.