[CMake] Modern cmake advise for transitive library dependencies

[Mario Emmenlauer]

Dear cmake team and user community,

I'd kindly like to ask for advice on how to handle transitive
dependencies cleanly with "modern" cmake. I'm often plagued by this
problem: I have a library X that optionally depends on library A.
When I build library Y that depends on X, how do I (cleanly) handle
the optional dependency A?
Assume "A" would be "Qt5Core". I can link X publicly against Qt5::Core.
But when I import X into Y, then Y will complain about unknown target
Qt5::Core, unless I do find_package(Qt5 COMPONENTS Core) in Y. But since
Qt is optional in X, I would need to track somehow if Qt was enabled
before.

But what good is the public transitive dependency when I manually need
to track it, to find the libraries again? Am I missing something?
Can the dependency be automatically resolved, or can I query some
variable if X was linking A?

How to do this cleanly?

Thanks a lot and all the best,

     Mario Emmenlauer