Package Configuration Harmonization, master branch (2021.10.19.)

Inspired by the inconveniences around !872 (merged) and !877 (merged), I am trying to make the external project builds a bit more configurable...

• I introduced CACHE variables for the source code in all of the package, to make it possible to override these in the atlas/athena build scripts, if necessary.
• Added double-quotes around all the path name occurrences, to make the code more friendly towards path names with spaces in them.
• Removed any outdated references to ATLAS_RELEASE_MODE. (Which has been removed from the code a good while ago.)

Note that the amount of changes differed package-by-package. Since some of the "more recent" packages needed very little work, while the "older" ones required quite a bit.

In the end this turned out to be a way bigger update than I originally thought. ☹ So the CI will have to do some real work, as I didn't test the changes under all the possible circumstances...