[raster] Cannot assume only one Python on system

[bnordgren]

Corrolary: makefiles and configure need to allow user to specify python executable.

PostGIS with raster support does not build on arch linux without modification at the moment, as they have made the "python" executable be python 3 and the "python2" executable be python 2. GDAL python bindings are installed only for Python 2.

In general, however, it will probably not be unusual to find a python 3 and a python 2 on the same system with varying ways to cope with the name collision. THe build scripts need to allow the user/packager to specify how this should be handled in their environment.

[dustymugs]

Python no longer a dependency for PostGIS Raster.