Externals Update, master branch (2019.01.22.)

This is to update to atlasexternals-2.0.21.

The full list of changes wrt. 2.0.20 is technically a lot (atlasexternals@2.0.20...2.0.21), though conceptually it's a relatively short list.

• Introduced fixes for AnalysisBaseExternals on Ubuntu and macOS. This was cherry picked from 1.0, where I'm trying to revive the analysis release builds for these platforms.
• Added FindCURL.cmake to AtlasLCG to be able to use CURL from the LCG releases if necessary. This I made for ATLINFR-2820 originally, but in the end !20440 (merged) did make this update unnecessary. Still, it's not a bad update per se.
• Updated the code handling Doxygen to work successfully with LCG_94. This is to address ATLINFR-2883.
• Added a lot of CMake changes to make the code work smoothly with CMake >=3.13. (atlasexternals!418 (merged))

It's the last point which is truly non-trivial. I made a lot of changes in the central code to resolve some long standing issues. This required changing a variable naming convention (telling FindFoo.cmake where to look for Foo using FOO_ROOT) that I used in that code since the very beginning. This also means that to incorporate this update in the externals, I also had to update a number of files in this repository. These changes are in all cases very trivial. Though quite numerous... 😛

I've been testing these changes for about a week now, still, since they are significant, it's not impossible that I may have missed something. Let's see what the CI tells us...