Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Turned off the build of (most of) the static Boost libraries, keeping only the dynamic ones; - Fixed the dictionary error in the BAT build that showed up when that build was switched to be performed out-of-source; - Added KLFitter to the build of AnalysisBaseExternals and AthAnalysisExternals.