Improve unused sources detection and fix warnings

!3334 (merged) introduced detection of unused subdirectories and source files but the DD4hep build relies (at the moment) on exclusion of subdirs and files to avoid building DetDesc specific code.

This MR introduces a mechanism to flag subdirectories as explicitly ignored (to be backported to run2-patches, !3357 (merged)) and fixes the warning about unused files (for source files the logic detects directories partially used but it's OK if all .cpp files in a directory are not used).

To be noted that TGeoTransportSvc was probably lost in the rebase of !2931 (merged) and required some intervention (f2ccd704) to actually compile.

I also fixed an issue with CMake < 3.19.

Closes #171 (closed)