cmake: fix find Threads

[abique]

For reference: microsoft/vcpkg#44940

[dg0yt]

This complements

flac/src/libFLAC/CMakeLists.txt

Lines 111 to 113 in 8d64845

	if(HAVE_PTHREAD)
	target_link_libraries(FLAC PUBLIC Threads::Threads)
	endif()

.

[Croydon]

Related issue: #820

[ktmf01]

I'm not fluent in CMake, so please help me out here. This dependency is added when the ENABLE_MULTITHREADING input is ON, but would using the output variable HAVE_PTHREAD be better? This is suggested in #820

[dg0yt]

If your library links Threads::Threads, this is exposed in the exported CMake config, and the CMake config must provide it. Basically, the config mirrors build-time find_package with usage-time find_dependency (minus the REQUIRED, but this is a different topic).

So this is what this PR does. Canonical, minimal CMake, mirroring CMakeLists.txt.

#820 is different:

• It uses a different condition. I don't know which condition is used in CMakeLists.txt. But the thread CMake module, and Threads::Threads, is generally also used to represent Windows threads. @HAVE_PTHTREAD@ doesn't look like it would represent this case.
• It sets THREADS_PREFER_PTHREAD_FLAG. I'm not sure if this belongs to a CMake config. It may change the properties of Threads::Threads, but this might be for the project to decide.
• It sets CMAKE_THREAD_PREFER_PTHREAD. AFAIU this is obsolete now, removed in CMake 3.14.

[dg0yt]

BTW this change is applied in the vcpkg port.

[ktmf01]

Would that mean that the wrong variable is used in the lines you quoted last time?

flac/src/libFLAC/CMakeLists.txt

Lines 111 to 113 in 8d64845

	if(HAVE_PTHREAD)
	target_link_libraries(FLAC PUBLIC Threads::Threads)
	endif()

Also, I'm wondering what would happen if find_package(Threads) doesn't find any multithreading capability. Then the build would not require a lib, but the file would say it does. Maybe we shouldn't check for HAVE_PTHREAD or ENABLE_MULTITHREADING, but instead for Threads_FOUND?

Finally, and I understand this is not something FLAC-specific, how does this work if configuration during build finds one library (for example pthread), and configuration with that flac-config.cmake file finds another multithreading capability. Would that even be possible?

[dg0yt]

Would that mean that the wrong variable is used in the lines you quoted last time?

I deliberately said "different", not "wrong". The full context needs to see

flac/CMakeLists.txt

Lines 225 to 236 in afb801b

	if(ENABLE_MULTITHREADING)
	set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
	set(THREADS_PREFER_PTHREAD_FLAG TRUE)
	find_package(Threads)
	if(CMAKE_USE_PTHREADS_INIT)
	set(HAVE_PTHREAD 1)
	else()
	if(HAVE_THREADS_H)
	set(HAVE_C11THREADS 1)
	endif()
	endif()
	endif()

So, HAVE_PTHREAD might indeed be more suitable to mirror the actual wiring in the CMakeLists.txt files. (The option "ENABLE_MULTITHREADING" is not directly wired to the use of Threads::Threads, and it may not even enable multithreading.)