[Fix] build: Autotools: Change detection of mingw-std-threads to use …

…mingw.mutex.h instead of mingw.thread.h because GCC11 broke mingw-std-threads' thread implementation. See <meganz/mingw-std-threads#79> and <https://forum.openmpt.org/index.php?topic=6822>.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@17309 56274372-70c3-4bfc-bfc3-4c3a0b034d27

build/autotools/configure.ac

@@ -56,7 +56,7 @@ AX_PTHREAD([
 AC_LANG_PUSH([C++])
 case $host_os in
  mingw32*)
-  AC_CHECK_HEADER([mingw.thread.h],
+  AC_CHECK_HEADER([mingw.mutex.h],
    [
     have_mingwstdtthreads=1
    ],