Make CMake artifacts configuration mergeable.

CMake artifacts (everything with *.cmake) always needs to be merge-able independent of configuration. This means that an installation with CMAKE_BUILD_TYPE=Release and an installation with CMAKE_BUILD_TYPE=Debug should be able to merge the *cmake files in a common directory structure.
Unfortunately the current design of QtBuildInternalsExtra.cmake.in does not allow it since in QtPostProcessHelpers.cmake variables are added to the build that are configuration dependent! As such QtBuildInternalsExtra.cmake from a Debug and a Release build are not equal and not merge-able!