Rather than have the python literal string, use CMake's find_package
to find actual python executable. This is more useful if you have
both Python 2 and 3 installed. CMake currently only supports python 2
and so will automatically find that version
Squashed commit of ca128f7dcd28cbcfba154c8577ed54d4aa71dd02 with
contributions from Mark Constable (markc@renta.net) and Daniel Gröber
(darklord@darkboxed.org).