Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Introduced the LCG_IGNORED_SYSTEM_PATHS and LCG_SYSTEM_IGNORE_PATH variables for blacklisting selected directories when searching for externals; - Various fixes meant for macOS; - Fixed Davix-0.7.1's build in the absence of a python interpreter; - Added some fixes to how Doxygen is used by atlas_project(...).