So far, SalYieldMutex was declared/defined in the same header and source as SalGenericInstance, and only built when building headless code is enabled. Move it from there to an own header and source file and remove the restriction to only build the code when headless code is enabled. This prepares for using the class in the qt6 VCL plugin also on Windows. (WIP change to add qt6 support for Windows: [1]) [1] https://gerrit.libreoffice.org/c/core/+/199615 Change-Id: Ic7152b43b42fc49b749213593637473e8a19de97 Reviewed-on: https://gerrit.libreoffice.org/c/core/+/202281 Tested-by: Jenkins Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>