Tagging the following updates for 21.X: - Made the BAT build not rely on RooStats. To make it a bit easier to use against custom-built ROOT versions. - Updated the way status messages are printed while the `atlas_project` function reads in the configuration of all packages. To make the printouts in a partial build less overwhelming. - Updated the ROOT build to use its built-in GSL version. - Updated AthAnalysisExternals and AthSimulationExternals not to build code in a multi-threaded way.