Project Export Config Improvements, master branch (2022.02.17.)

Made sure that THREADS_PREFER_PTHREAD_FLAG would be set earlier than it is now, trying to fix ATLINFR-4377. Unfortunately these changes by themselves do not fix that issue (I will make a separate MR into atlas/athena for that), but since I made all this cleanup already, I thought I might as well merge it in...

• Introduced PreConfig.cmake files for each project, which would set up the cache variables that the projects want to set for whoever uses them.
• Tried to harmonise the settings along the way. This included removing (hopefully) unnecessary settings in AthAnalysisExternals and AthGenerationExternals.
  • Removing add_definitions(-DATHENAHIVE) from the AthAnalysisExternals configuration I'm not too worried about, that was just left in as a goofy mistake;
  • I am however a little more worried about the BLA_STATIC setting in AthGenerationExternals that I removed. 🤔 @ewelina, I'm pretty sure that with the way that we pick up the libraries from LCG these days, this setting is not needed. And even if it is, it should be set by AthGeneration, it shouldn't be set here. Still, we should remember this change, in case you notice anything unexpected in the master branch...

Pinging @fwinkl for info. 😉