FindThreads: fix checking for CMake::Threads target
Merge topic 'FindThreads_overhaul' into next

993c41d4 FindThreads: fix checking for CMake::Threads target
FindThreads: introduce an imported target to link to

This not only holds the library, but can also hold compiler flags needed, e.g.
the -pthread flag preferred by gcc on some platforms. There was no clean way
to get that compiler flag from the module until now.
FindThreads: introduce THREADS_PREFER_PTHREAD_FLAG (#14767)
Merge topic 'FindThreads_overhaul' into next

5838043e FindThreads: introduce THREADS_PREFER_PTHREAD_FLAG (#14767)
decc7d1e FindThreads: introduce an imported target to link to