RFC: Using CMAKE_VERIFY_INTERFACE_HEADER_SETS

[Albert Astals Cid]

Now that we have a newer cmake we can use CMAKE_VERIFY_INTERFACE_HEADER_SETS

It allows to create a test to check that each of the headers is correctly installable on its own.

There is a MR of what the change entails at 
https://invent.kde.org/frameworks/karchive/-/merge_requests/80

You can see a pipeline where the test proves it works at
https://invent.kde.org/frameworks/karchive/-/pipelines/1134293

I had modified the code to compile correctly but so that headers would not be standalone includeable, see the code at 
https://invent.kde.org/frameworks/karchive/-/merge_requests/80/diffs?commit_id=6c3d669f6f49fa38450a8658ff52f8d0e36fe620

Should we add this to all frameworks?

I would need some ECM expert to see if we could make it so less code needs to be repeated.

I guess the CMAKE_VERIFY_INTERFACE_HEADER_SETS can go to the KDEFrameworkCompilerSettings ?

But what about the add_test?

Cheers,
  Albert