Make: Fixed Python version issue

Makefile uses the `python` command by default, and the python code uses Python 2 syntax (specifically, Python 2's`print "..."` vs  3's `print("...")`).

This pull request fixes this by adding a `PYTHON` Makefile variable, which is platform dependent, although set to `python2` on all platforms for now. (Made platform dependent in case an untested platform uses another executable than python.)

Tested on Linux, Windows, and Mac OSX.