Gaudi Fixes, master branch (2021.01.14.)

These are various fixes/improvements to make it possible to use a custom build of Gaudi as part of a WorkDir based project build.

The idea is, that once these fixes are in the nightly, it will be possible to use a custom build/installation of Gaudi as part of a "partial build" like:

Gaudi_DIR=<somewhere>/gaudi_install cmake -DATLAS_PACKAGE_FILTER_FILE=../athena/Projects/WorkDir/package_filters_example.txt ../athena/Projects/WorkDir/

But for the moment, while the updates for FindGaudi.cmake and LCGFunctions.cmake are not in the nightly yet, one must do the following:

Gaudi_DIR=<somewhere>/gaudi_install cmake -DATLAS_PACKAGE_FILTER_FILE=../athena/Projects/WorkDir/package_filters_example.txt -DCMAKE_MODULE_PATH=<somewhere>/atlasexternals/External/Gaudi/cmake -DAtlasCMake_DIR=<somewhere>/atlasexternals/Build/AtlasCMake -DLCG_DIR=<somewhere>/atlasexternals/Build/AtlasLCG -DAtlasHIP_DIR=<somewhere>/atlasexternals/Build/AtlasHIP ../athena/Projects/WorkDir/

Pinging @fwinkl and @leggett.

P.S. @clemenci, we should think about adding some "helper variables" to the Gaudi configuration for this. Like I do here for the ATLAS projects:

• https://gitlab.cern.ch/atlas/atlasexternals/-/blob/master/Build/AtlasCMake/modules/skeletons/ProjectConfig.cmake.in#L23-31
• https://gitlab.cern.ch/atlas/atlasexternals/-/blob/master/Build/AtlasCMake/modules/AtlasFunctions.cmake#L477-487