Doxygen Dependency Fixes, 1.0.48-patches branch (2020.10.06.)

This is meant to fix an issue that the 21.X branches encountered on the x86_64-centos7-gcc62-opt platform. (Against LCG_88.)

It turns out that the incorrect Cairo dependency was missing from AthDerivation-21.2 because FindDoxygen.cmake is not setting up Cairo in the 1.0 branch anymore. I cherry-picked the commit that removed that dependency, into this branch as well.

At the same time I was not actually aware of the name change between Cairo and cairo. I now updated FindCairo.cmake to handle this name change correctly. (Which I plan to cherry-pick into all other active branches as well.)

Thanks to @jchapman and @boeriu for making me aware of this!