enable environment variable to override gdal-config --prefix as root directory of local GDAL installation

When installing GDAL on OS/X using one of the frameworks from here: http://kyngchaos.com/software/frameworks, the resulting installation places libraries and header files in a location that does not correspond to the value returned by gdal-config --prefix. This causes future installation of pygdal to fail, since it can't find the headers and libraries necessary to compile osgeo.

This PR enables the installation script to handle overrides to the GDAL root directory, as manipulated via an environment variable (GDALHOME)