Externals Update, 21.2 branch (2021.07.14.)

Updated all projects to atlasexternals-1.0.76. The changes wrt. atlasexternals-1.0.71 are plentiful (atlasexternals@1.0.71...1.0.76):

• Made FindCairo.cmake work with >=LCG_92;
• Stopped the (standalone) HDF5 build from installing PkgConfig files;
• Switched the yampl build to downloading the source as a tarball instead of getting it from GitHub;
• Added FindApfel.cmake to AtlasLCG;
• Added a fix for a very rare race condition in the build of the PyAnalysis package;
• Added FindHto4l.cmake and FindProphecy4f.cmake for finding those two MC generators in an LCG release;
• Taught FindSuperchic.cmake to make the superchic executable, and the $SUPERCHICPATH environment variable available in the release's runtime environment;
• Introduced Findlibzip.cmake into AtlasLCG;
• Updated KLFitter to version 1.4.0. (Pinging @kzoch and @tdado.)

However it's only the last update, the one switching to KLFitter 1.4.0, that is relevant for 21.2.

@amehta, @dshope, I believe we should wait for this update with tomorrow's release build. Hopefully it will go through still today, but if not, I think it would be worth delaying the build by 1 day for the KLFitter update.

Note that I'm not using full-build on the MR, only full-unit-tests. Since build-wise the update should be possible to handle with an incremental build...