Better handling of unknown PYTHON_VER in Makefile

If PYTHON_VER is not known to the Makefile, don't fall back to
PYTHON_MINOR of 2.7.3, instead ask the user to specify a correct
PYTHON_MINOR explicitly.  Otherwise you get weird failures like #203.

Also allow the user to override PYTHON_MINOR on the `make` command line,
e.g.

    make PYTHON_VER=3.3 PYTHON_MINOR=3.3.6