Bump mingw-std-threads submodule (LMMS#6817)

* Bump submodule and generate std headers

* Commit submodule update

* Downgrade to earlier commit
Can be upgraded once a proper C++20 MinGW compiler
is available for the docker images.

* Downgrade to the correct commit

* Append mingw_stdthreads to EXTRA_LIBRARIES
Currently for this CMake version, it seems that there is no support to
link other targets to object libraries. I'll add it to EXTRA_LIBRARIES
instead.

* Add LMMS_ prefix to USE_MINGW_STD_THREADS

* Use built-in MINGW CMake variable

* Use lowercase rather than uppercase

src/3rdparty/CMakeLists.txt

@@ -7,6 +7,12 @@ ADD_SUBDIRECTORY(hiir)
 ADD_SUBDIRECTORY(rpmalloc)
 ADD_SUBDIRECTORY(weakjack)
 
+if(MINGW)
+	option(MINGW_STDTHREADS_GENERATE_STDHEADERS "" ON)
+	add_subdirectory(mingw-std-threads)
+	set(LMMS_USE_MINGW_STD_THREADS ON PARENT_SCOPE)
+endif()
+
 # The lockless ring buffer library is compiled as part of the core
 SET(RINGBUFFER_DIR "${CMAKE_SOURCE_DIR}/src/3rdparty/ringbuffer/")
 SET(RINGBUFFER_DIR ${RINGBUFFER_DIR} PARENT_SCOPE)

src/CMakeLists.txt

@@ -170,6 +170,10 @@ if(LMMS_HAVE_MP3LAME)
 	list(APPEND EXTRA_LIBRARIES mp3lame::mp3lame)
 endif()
 
+if(LMMS_USE_MINGW_STD_THREADS)
+	list(APPEND EXTRA_LIBRARIES mingw_stdthreads)
+endif()
+
 SET(LMMS_REQUIRED_LIBS ${LMMS_REQUIRED_LIBS}
 	${CMAKE_THREAD_LIBS_INIT}
 	${QT_LIBRARIES}