Tag to be used with the 21.X branches of atlas/athena.
Updates in this tag:
  - Added FindHJets.cmake for finding the HJets generator;
  - Fixed the generation of .d files for Ninja, to make dictionary generation behave correctly with this build tool;
  - Added some extra find-modules to AtlasLCG;
  - Stopped setting up the Cairo and Pango packages when using graphviz.

Note that this tag has problems with building LCG based releases. Some of the updates backported into this tag from the master branch were not done quite correctly. All in all, this tag should not be used...