WIP: First shot to introduce SYSTEM property for target

This aims to allow the consumption of 3rd-party libs as SYSTEM libraries. This will enable developers to use higher warning levels with their own code without getting lost in noise.

TODO:

• Check that the property can be set on the target.
• Make it work (generator must emit -isystem instead of -I)
• How can the property be set on ALIAS targets?
• Add tests
• Add documentation
• Add keyword to add_subdirectory (probably in a different PR)

If a SYSTEM keyword is added to add_subdirectory similar to EXCLUDE_FROM_ALL there should be no need to set the property for ALIAS targets.

Issue: #18040 (closed)
Topic-rename: target-SYSTEM