Follow-up from ""Modernize" CMake configuration"

The following discussion from !79 (merged) should be addressed:

• @rmatev started a discussion: (+1 comment)

  clang doesn't like the new cmake for some reason: https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/1678/Geant4/x86_64_v2-centos7-clang12-opt/build

  any ideas?