WIP: Externals Update, 21.2 branch (2019.03.12.)

Updated all projects to atlasexternals-1.0.49. The full list of changes (atlasexternals@1.0.48...1.0.49) is:

• Added FindHJets.cmake for finding the HJets generator (meant for the 21.6 branch, should not affect this one);
• Fixed the generation of .d files for Ninja, to make dictionary generation behave correctly with this build tool (thanks to @fwinkl);
• Added some extra python find-modules to AtlasLCG;
• Stopped setting up the Cairo and Pango packages when using graphviz.

The last one is meant to get emacs to work once again in the AthAnalysis/AthDerivation runtime environment. Note that this should make it unnecessary to rely on the alias set up by asetup. Though keeping that alias around will not hurt.