Allow user to specify a python executable.

Fall back on `python`. Unlike the solution in PR #2395 this puts all the responsibility for selecting the right python in the case that `python` is Python 3 on the user. I've followed the lead of the Python documentation Makefile. See:

http://hg.python.org/cpython/file/340d48347295/Doc/README.txt
http://hg.python.org/cpython/file/340d48347295/Doc/Makefile

Intent is that if your `python` is Python 2 everything works as before and that is your `python` is Python 3 but you have a `python2` you can specify that instead.

No changes have been made to the shebangs in the various Mapnik python scripts. But if they're run by the Makefile it'll all work out.