Tag to be used with the master branch of atlas/athena.
Updates since 2.0.15:
  - Updated the CLHEP and G4 builds to properly clean up when switching to a different version of CLHEP/G4. Allowing for CI friendly version updates of these two externals.
  - Updated the ROOT dictionary building CMake code to better detect when Reflex dictionaries need to be re-generated in an incremental build.