We currently apply a patch https://github.com/cms-sw/cmsdist/blob/c1e2d7ab02f957e01e7325ecf1259eb8f06b0e8d/dd4hep-add-ppc64-macro-check.patch so that we get into that condition and include that header when building on ppc64. So if it's reasonable I propose to get this in

https://github.com/Geant4/geant4/blob/v10.3.0/source/global/management/include/G4Version.hh#L49

and print error message if that is not the case.