Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Updated AtlasLCG and AtlasCMake to use C++17 for GCC >=7; - Updated XRootD, KLFitter and BAT to build successfully with GCC 8 in C++17 mode; - Turned off the build of CheckerGccPlugins with GCC >=7; - Fixed a mistake in the CLHEP build that caused a build error with new CMake versions; - Updated FindGaudi.cmake to behave just a little bit better than it did before.