Externals Update, master branch (2021.05.31.)

Upgraded all projects to atlasexternals-2.0.108. The list of changes wrt. atlasexternals-2.0.107 is the following (atlasexternals@2.0.107...2.0.108):

• Updated CheckerGccPlugins to handle atomics more reliably, and to ignore the unwind.h and XercesC headers (thanks to @ssnyder);
• Introduced Finddistro.cmake into AtlasLCG (thanks to @elmsheus).

The latter is a follow-up from !43741 (merged) and !43936 (merged), and is meant to fix ATLINFR-4119.

With Finddistro.cmake available, I re-enabled the use of the distro module in the code. I just took @elmsheus's code from !43741 (merged) manually, just because this was the simplest to do. (Cherry-picking the changes would've been more trouble than it was worth...)

Since all of these are relatively minor updates, there's no need for either full-build or full-unit-tests on this one.