Windows Python Makefiles: use PYTHON variable

This makes it marginally easier to override PYTHON to e.g.
C:\Python34-x86\python if cross-compiling.
Remove Python Makefile.old files

These have significantly diverged from the actual Makefile.vc build
files, so there's no reason to keep these around anywhere except in
version history.
Python 2 + 3 setup.py: small reformatting/refactoring

* Use os.path.join instead of manually concatenating filesystem paths
* Reformat distutils.core.setup call
* Remove unused imports
* (2) Close setup.ini file handle after using it
* (3) Move subprocess.check_output(...).strip().decode() behavior into a
  shared function