[CMake][Exp PyROOT] Build only one PyROOT version if either PYTHON_EXECUTABLE or Python_EXECUTABLE are passed
[CMake][Exp PyROOT] Build only one PyROOT version if either PYTHON_EXECUTABLE or Python_EXECUTABLE are passed

PYTHON_EXECUTABLE or Python_EXECUTABLE are specified: only one PyROOT is
built with the version specified;

Python2_EXECUTABLE and/or Python3_EXECUTABLE are specified: both
versions are built

Nothing is specified: both versions are built

root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root