cmake: Workaround for upstream flac-config.cmake not creating Threads::Threads

Upstream issue: xiph/flac#820

Author: madebr

CMakeLists.txt

@@ -578,6 +578,12 @@ if(SDLMIXER_FLAC_LIBFLAC)
             if(NOT SDLMIXER_FLAC_LIBFLAC_SHARED)
                 list(APPEND PC_REQUIRES flac)
             endif()
+            # Xiph's CMake script does not handle Threads::Threads correctly:
+            # https://github.com/xiph/flac/issues/820
+            get_property(flac_interface_link_libraries TARGET FLAC::FLAC PROPERTY INTERFACE_LINK_LIBRARIES)
+            if(Threads::Threads IN_LIST flac_interface_link_libraries)
+                find_package(Threads)
+            endif()
         else()
             message(${fatal_error} "libflac NOT found")
         endif()

cmake/SDL3_mixerConfig.cmake.in

@@ -74,6 +74,12 @@ if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/SDL3_mixer-static-targets.cmake")
 
         if(SDLMIXER_FLAC_LIBFLAC AND NOT TARGET FLAC::FLAC)
             find_dependency(FLAC)
+            # Xiph's CMake script does not handle Threads::Threads correctly:
+            # https://github.com/xiph/flac/issues/820
+            get_property(flac_interface_link_libraries TARGET FLAC::FLAC PROPERTY INTERFACE_LINK_LIBRARIES)
+            if(Threads::Threads IN_LIST flac_interface_link_libraries)
+                find_package(Threads)
+            endif()
         endif()
 
         if(SDLMIXER_MOD_XMP AND NOT TARGET libxmp::libxmp)