build: add python_binary=xxx scons option
build: add python_binary=xxx scons option

debian: python2.7
gentoo: python-2.7
Not sure how to auto-guess those.

achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint