SITEROOT Setting Fixes, master branch (2021.01.27.)

Made it possible to use SITEROOT != "/afs/" during the build. The CMake code was unfortunately not acknowledging the setting of that variable in the build environment before now. 😦

These are necessary to have the master--dev3LCG and master--dev4LCG nightlies "remember" where they should pick up data files from at runtime, without the user having to specify it himself/herself, as discussed at length in:

This should also allow us to fix ATLINFR-3946. (After some further tweaks in the Jenkins jobs.)

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