Add a check using ROOT_CXX_STANDARD

[jmcarcell]

BEGINRELEASENOTES

• Add a check for the C++ standard using ROOT_CXX_STANDARD that was introduced yesterday in ROOT (root-project/root@d487a42)

ENDRELEASENOTES

currently nightly builds with HEAD of ROOT are not working in the LCG stacks. I think it breaks because of this removal:
root-project/root@d487a42#diff-99952e2ce9de0fe836877b1910798d5d4c60c7c27a62c1b51410643a6a2c9654L180

Building this PR with a ROOT version with ROOT_CXX_STANDARD set is working for me.

[tmadlener]

Do we already have a check somewhere that makes sure that building podio with c++20 also requires root to have c++20?

[tmadlener]

Does this fixe this error? https://github.com/AIDASoft/podio/actions/runs/7555260579/job/20569847736?pr=383#step:4:170

[jmcarcell]

I added a check (prints a warning) when the C++ standard is different in ROOT from the required. It could be an error but it was somewhat useful to build some packages with C++20 when we didn't have ROOT with C++20 to see if they failed or not; anyway mismatches can't happen in spack so in user builds there would be a visible warning.

Does this fixe this error? AIDASoft/podio/actions/runs/7555260579/job/20569847736?pr=383#step:4:170

Yes

[tmadlener]

Thanks for the clarification. Merging this to unblock a few of the other PRs that are essentially ready.