cmake: Prefer -lpthread for now because clang++ and VLC

It seems that clang doesn't support -pthread at the linking stage, but
only at the compilation, so putting it into the .pc file causes issues.

CMakeLists.txt

@@ -358,7 +358,6 @@ elseif (WIN32)
 		message(FATAL_ERROR "Failed to find pthread library. Specify PTHREAD_LIBRARY.")
 	endif()
 else ()
-	set(THREADS_PREFER_PTHREAD_FLAG ON)
 	find_package(Threads REQUIRED)
 	set(PTHREAD_LIBRARY ${CMAKE_THREAD_LIBS_INIT})
 endif()