Fix compilation of preprocessed header for functors

Rosen Matevrequested to merge
rm-functor-cmake into master

The current logic to split includes in system and local ones using generator expressions was failing when $PROJECT_SOURCE_DIR contained regex control characters (such as + as in gcc13+dd4hep).

This bug was exposed by the folloing change in lb-stack-setup: rmatev/lb-stack-setup@0f387993

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