Small fixes for boost-mpi3

Beojan Stanislausrequested to merge
bstanisl/atlasexternals:mpi-fixes into main
  • Fix failing CI in athena!73723 (merged) by adding boost-mpi3 to AthGenerationExternals and AthAnalysisExternals
    • Necessary because MPIEventLoopMgr is in AthenaServices (and splitting it out would require making the AthenaHiveEventLoopMgr header public)
  • Add a couple of extra environment variable exports to FindMPI wrapper for the LCG OpenMPI use case

Merge request reports