On dashmacmini2 the test showed output like this:

-- Found PythonInterp: /usr/bin/python (found version "Unknown option: --
usage: /usr/bin/python [option] ... [-c cmd | file | -] [arg] ...
Try `python -h' for more information.")

On my machine where python outputs "Python 2.7" this worked, but
PYTHON_VERSION_MAJOR, PYTHON_VERSION_MINOR, and PYTHON_VERSION_PATCH were all
set to "2.7".

Add some checks that the version output has the expected form before using it.