Made Athena explicitly set up the LCG release itself.

In order to be able to find Qt5 "early enough" it's not possible
anymore to wait for AthenaExternals with setting up the LCG release,
as it used to happen so far...

Projects/Athena/cmake/PreConfig.cmake.in

@@ -9,15 +9,25 @@ set( ATLAS_FLAKE8 "@ATLAS_FLAKE8@"
 set( ATLAS_PYTHON_CHECKER "@ATLAS_PYTHON_CHECKER@"
    CACHE STRING "Python checker command to run during Python module compilation" )
 
-# Due to the way CMake handles propagation of targets to dependees,
-# any "external" that is used through imported targets in the build need
-# to be imported explicitly here.
+# Figure out whether to use QUIET in the following calls.
 set( _quietFlag )
 if( Athena_FIND_QUIETLY )
   set( _quietFlag QUIET )
 endif()
+
+# Set up the correct version of LCG. While AthenaExternals 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@ )
+find_package( LCG @LCG_VERSION_NUMBER@ REQUIRED EXACT ${_quietFlag} )
+
+# Due to the way CMake handles propagation of targets to dependees,
+# any "external" that is used through imported targets in the build need
+# to be imported explicitly here.
 find_package( Acts ${_quietFlag} COMPONENTS Core )
 find_package( nlohmann_json ${_quietFlag} )
 find_package( Qt5 ${_quietFlag} COMPONENTS Core OpenGL Gui PrintSupport Widgets
    Sql Network )
+
+# Clean up.
 unset( _quietFlag )