Add a check for the standard that Geant4 was compiled with

[jmcarcell]

BEGINRELEASENOTES

• CMake: Add a check for the c++ standard that Geant4 was compiled with, and fail if it is
  different from the one required for DD4hep.

ENDRELEASENOTES

I think failing is the correct behavior instead of a warning, for example. The reason is that there will be some files that will be compiled with the same flags that were used to compile Geant4, which means that it will overwrite whatever version of the standard one is requiring for DD4hep (at least with gcc, who is happy to get two different -std=c++XX flags).

[github-actions]

Test Results

       6 files         6 suites   6h 42m 25s ⏱️
   356 tests    352 ✔️ 0 💤 4 ❌
1 058 runs  1 054 ✔️ 0 💤 4 ❌

For more details on these failures, see this check.

Results for commit 91808aa.