In order to compile client code (or partially compile packages of this repository) against an AnalysisBase release/nightly, all imported targets used by the targets exported by AnalysisBase must be available.
This addition takes care of that, providing the Threads::Threads imported target to the CMake configuration of AnalysisBase.
Note that in the 21.2 branch this is instead done in the post-configuration file. (https://gitlab.cern.ch/atlas/athena/-/blob/21.2/Projects/AnalysisBase/PostConfig.cmake.in) But experience with imported targets and partial builds in master taught me that it's more robust to do it during "pre-configuration" instead.