GCC Clang Pragma Warning Fix, 23.0 branch (2023.07.10.)

As it turns out (see !64172 (comment 6900052) and https://bigpanda.cern.ch/nview/?nightly=main--dev4LCG_Athena_x86_64-el9-gcc13-opt), GCC is very touchy about seeing pragmas that are not meant for it. So we need to only expose these pragmas to the ROOT dictionary generation. Even Clang as the main C++ compiler does not need to see them.

This is a follow-up to !64172 (merged). But I still don't know how I missed these while writing that MR. 😕

🤞 Let's hope that this will be the end of this issue (ATLINFR-4996)... Pinging @christos and @elmsheus.