Use python2 (if available) to run cxxtestgen
As python 2.x is reaching EOL soon (PEP 373) and some distros already switched to use python 3.x as a default python interpreter, I propose to follow PEP 394 and use python2
command/link instead of python
when invoking cxxtestgen.
If python2
is missing, CMake will fall-back to using python
(if it's on $PATH
).