Use all distutils config files in pyximport

Pyximport uses distutils to compile cython code. In my project,
I needed special compiler options to compile the code, so I wrote
a setup.cfg file to cope for that. Weirdly, pyximport completely
ignored this. So I tried to find where pyximport is looking, and
found: nowhere! This patch removes this oddity.

The proponents of the current behavior argued that users might
be astonished to see that their settings were overridden by a random
config file. This argument is flawed: config files don't just appear by
themselves. And for those people out there who know how to write config
files and know where to put them, let them do it!