Externals Update, 21.2 branch (2019.07.12.)

Updated all projects to use atlasexternals-1.0.53. This was necessary to fix the build issue that last night's x86_64-centos7-gcc8-opt nightlies ran into. For a longer explanation of the issue, see ATLINFR-3120.

The full update can be seen in: atlasexternals@1.0.52...1.0.53. Which is basically just this MR: atlasexternals!513 (merged). All in all it's just a few tweaks to how AnalysisBaseExternals and AtlasLCG look for externals on the build machine.