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

These changes are here to address (some of) the build issues encountered in !393 (merged).

I only made the changes in CLHEP to make my local build a little nicer. That's just updating the implementation to the style that I like to use these days.

The big change is in the Geant4 package. I introduced a list of build "sub-steps" to that target.

Unfortunately it turned out that the "forcedownload" step that I use on projects that build tar files downloaded from a webpage, is not appropriate for a git repository. I had to find a slightly different incantation with the git repo...

@jchapman, @bmorgan, @hgray and @japost, G4 should be updated to a new version in the following way from now on:

• Update the git tag;
• Update the version number strings to use in the "patching";
• Modify the text printed by the Geant4-forcedownload step. (Add a period at the end, add an empty space at the end, etc.)

This new forcedownload step, coupled with the cleansource step (which itself only runs when CMake decides to re-run the forcedownload step, i.e. when you modify its printed message) are taking care of re-downloading the sources of Geant4 from scratch. So that the patch step could be done on top of the fresh git clone successfully.

Now let's see what the CI thinks about it. (I spent quite some time tweaking these options in an incremental setup...)