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.¬
Merge pull request #2406 from mapnik/user-specified-python

Allow user to specify a python executable.