[CMake][Exp PyROOT] Machinery to loop in parallel on multiple lists
[CMake][Exp PyROOT] Machinery to loop in parallel on multiple lists

In order to iterate in parallel over multiple lists (needed to build
targets and link libraries for multiple Python versions at the same time), we
introduce the machinery suggested in:

https://stackoverflow.com/questions/7932205/parallel-iteration-over-lists-in-makefile-or-cmake-file

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