Support for installing matplotlib-basemap in Hombrew locations other than /usr/local.

Hello,

I live with Homebrew in a location other than /usr/local. When trying to install matplotlib-basemap, this causes an error indicating that the GEOS library (which is installed through the respective homebrew package) can't be found:

    ==> python setup.py install --prefix=/opt/homebrew/Cellar/matplotlib-basemap/1.0.7 --record=installed.txt
    where geos is installed (for example, if geos_c.h
    is in /usr/local/include, and libgeos_c is in /usr/local/lib,
    set GEOS_DIR to /usr/local), or edit the setup.py script
    manually and set the variable GEOS_dir (right after the line
    that says "set GEOS_dir manually here".

To enable the package with an arbitrary `HOMEBREW_PREFIX`, this change in the formula worked for me.

Instead of simply doing a replacement in the `setup.py` as in this pull request, one could also use a proper patch as part of the formula. I can rewrite the pull request if you prefer the latter.

Thanks for considering this,

   Christian.