CMake: use system include dirs if used as sub-project

[stertingen]

The code should not emit warnings if used as sub-project.
This PR adds CMake logic to detect if Lyra was added using add_subdirectory and declares the include directories as SYSTEM includes.

[jayeshbadwaik]

@stertingen Are you using CMake > 3.25, because since CMake 3.25, add_subdirectory provides an option to add SYSTEM which seems a much more robust way of doing things:

1. It will annotate all usages with SYSTEM therefore preventing bugs where maintainer forgets to annotate something.
2. It allows user to decide if they want the add_subdirectory as SYSTEM or not and does not force the decision.

If that solution is working for you, then we don't need this in lyra.