Header Relocatability, main branch (2024.06.27.)

As I had to discover in atlas/atlasexternals!1136 (merged), !326 (merged) did not manage to fix everything for the https://gitlab.cern.ch/GeoModelDev/GeoModel/-/tags/6.0.0 tag. 😦 !327 (merged) introduced a new package, GeoModelCppHelpers, which didn't follow the changes that I made in !326 (merged). ☹

Generally, the CMake configuration of the project should be cleaned up a bit. 🤔 Since right now one can make such mistakes in just too many places. Unfortunately I can't really volunteer to do this. At least not any time soon. 😦