AthSimulation CMake Fixes, main branch (2024.02.28.)

Make it possible to use AthSimulation(Externals) from CVMFS.

One has to tell AtlasLCG to not look for any "generator packages", and
one also has to make sure to set up LCG early enough during the
CMake configuration. So that building code against AthSimulation would
work as it should.

Projects/AthSimulation/CMakeLists.txt

-# Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
+# Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 
 # Set up the project.
 cmake_minimum_required( VERSION 3.6 )
@@ -22,6 +22,7 @@ set( ATLAS_PYTHON_CHECKER ${ATLAS_FLAKE8}
 find_package( AtlasCMake QUIET )
 
 # Find the base project(s):
+set( LCG_COMPONENTS "externals" CACHE STRING "LCG components to set up" )
 find_package( AthSimulationExternals REQUIRED )
 find_package( Gaudi )
 

Projects/AthSimulation/cmake/PreConfig.cmake.in

-# Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
+# Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 #
 # Pre-config script to propagate variables to downstream projects
 #
@@ -29,6 +29,13 @@ if( @CMAKE_PROJECT_NAME@_FIND_QUIETLY )
   set( _quietFlag QUIET )
 endif()
 
+# Set up the correct version of LCG. While AthSimulationExternals does this as
+# well, we need to do it explicitly here, to make the following find_package
+# calls find the packages correctly.
+set( LCG_VERSION_POSTFIX @LCG_VERSION_POSTFIX@ )
+set( LCG_COMPONENTS "@LCG_COMPONENTS@" CACHE STRING "LCG components to set up" )
+find_package( LCG @LCG_VERSION_NUMBER@ REQUIRED EXACT ${_quietFlag} )
+
 # Imported target(s) used by the project.
 find_package( nlohmann_json ${_quietFlag} )
 find_package( GTest ${_quietFlag} )