CMake: flac-config.cmake is missing find_dependency(Threads)

[z-s-e]

The cmake Package Configuration File should find all dependencies that the targets need, otherwise configuration fails.

Right now, doing find_package(FLAC REQUIRED) in a project fails, unless one uses the work-around of having find_package(Threads REQUIRED) beforehand, which is not so nice.

I believe all that needs to be done here is adding the following lines to flac-config.cmake.in after the include(CMakeFindDependencyMacro):

if(@HAVE_PTHREAD@)
    set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
    set(THREADS_PREFER_PTHREAD_FLAG TRUE)
    find_dependency(Threads)
endif()

[ktmf01]

#830 has another solution proposed. I'm not sure which one is nicer, and what the side-effects of each are. Could you have another look? Maybe you could add another PR for this solution in case you think this is better?

[z-s-e]

Sorry, don't have time to look into this too closely at the moment.

In general it makes sense to have the same library lookup code in your flac-config.cmake as you do in your CMakeLists.txt, as otherwise you might have subtly different behavior between building and using the lib. So I would think you want to set THREADS_PREFER_PTHREAD_FLAG as well at least, from a cursory look, though it could also be that it doesn't really matter in this case. Maybe try to find out why that flag is set to understand whether it is needed in flac-config.cmake as well...