Can't use Genex in GLOBAL_TARGETS

[robertmaynard]

Description

You can't use generator expression in the arguments to GLOBAL_TARGET, as it will get converted to something like:

if(TARGET $<IF:$<BOOL:${_RAPIDS_ENABLE_UNSTABLE}>,CCCL::cudax,>)
   .... 
endif()

Which is invalid as generator expression can't be used in an if statement. Instead just always pass optional targets in GLOBAL_TARGETS as the function already does an existince check.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.

[robertmaynard]

@nirandaperera FYI

[bdice]

Thanks!

[robertmaynard]

/merge

[nirandaperera]

Thanks @robertmaynard . TIL!