Suggestion for darwin changes

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 also included instructions for users of Fink and MacPorts (though
perhaps those should go in a readme somewhere, instead).

Here are the changes:

I would appreciate a review, so I can submit a pull request.

-- Russell