Externals Update, master branch (2019.01.15.)

This is mainly to fix ATLINFR-2871.

The update to atlasexternals-2.0.20 brings the following updates (atlasexternals@2.0.19...2.0.20):

• Introduced the LCG_IGNORED_SYSTEM_PATHS and LCG_SYSTEM_IGNORE_PATH variables for influencing globally what directories to ignore during the build and environment setup script generation;
• Fixed the RPM dependency problem introduced when FindTBB.cmake started setting up TBB_VERSION using tbb/tbb_stddef.h.

At the same time I took the opportunity to clean up the build scripts in the master branch a bit. I turned all -x arguments into ones that can be specified multiple times, and they would remember/use all the received arguments. This required a bit of BASH magic that I just "stole" off of StackOverflow...

I removed the -d options from the scripts that had it, as in the new implementation users wanting to debug our cmake configurations will be able to provide -x --trace to the build scripts instead.

Finally, I also removed the git version checks from the build scripts. That version check was added when we were still thinking of attaching "notes" to git tags that would be created by the build scripts. But since we never started doing that, the scripts can really operate with any version of git.

Cc-ing the main developers/users of these scripts: @ozenin, @smh, @fwinkl, @leggett.