Allow include of G4 headers with or without 'Geant4/' prefix
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The standard deployment of Geant4 required that Geant4 headers are included without the Geant4/ prefix (although they get installed in include/Geant4) so the merge of !79 (merged) requires that the client code is adapted.
This MR modifies the build of the current Geant4 (pre !79 (merged)) so that client projects (Gauss and Gaussino) can already switch to the new #include policy.