Suggestion for darwin changes

Hi Russell,

At present all people buliding matplotlib on Mac OS X must edit I have modified to make it work with Mac OS X

(“darwin”) for Apple’s python, python and presumably Homebrew

python (since that uses /usr/local).

I’ve never actually edited in order to build matplotlib using Apple’s python, and it has never been a problem for me. I guess it’s because system python uses the standard include locations (unlike fink and macports).