Cherry-pick "Merge branch 'G4ForceDownload-master-20181205' into 'master'" into 2.0.15-patches

Geant4/CLHEP Build Improvements, master branch (2018.12.05.)

See merge request !399 (merged)

(cherry picked from commit 2109a52e)

• 6e62fd78 Made a number of improvements to the G4 build configuration.
• 1d64c33c Changed how the G4 re-download/re-clone is implemented.
• bbd375a9 Cleaned the CLHEP build configuration slightly.
• 9e5817c8 Updated CLHEP to implement forcedownload in the same way as Geant4.

This is in order to implement the plan described in ATLINFR-2843.