Expose CXX standard in rootbench

[stwunsch]

We require an exposed CMAKE_CXX_STANDARD in rootbench. Currenlty, it's not there or only in the case of being builtin with root. The hotfix looks like this but does not cover correctly the case "standard > 14" with an external build of rootbench.

[eguiraud]

Duplicate of root-project/root#7644