Use a better way to find PYTHONHOME from the found python, to avoid guessing...

Hi @akraszna

First off ... I didn't mean to push to a branch in this repo, I should have sent this to my fork and didnt even expect to have permission to push here, but it's happened now, so hope we can continue here anyway ....

I was resurrecting the demonstrator project you created a while back for a stat software release and when running it on my local machine I noticed that if I told it not to build python as part of the release build then my local python installation was picked up but the PYTHONPATH it inferred from it's path (/usr/local/bin/python) would then cause problems because that was just a symlink to the real location of python. The fix was using a snippet of cmake from the original FindPython module which infers the path by calling python.

Do you think this is a sensible improvement to the code?