Log Message:
libopenmpt: update to 0.6.4.

### libopenmpt 0.6.4 (2022-06-12)

 *  [**Bug**] openmpt123: Linking failed when using Autotools build system
    targeting MinGW.
 *  [**Bug**] tests: Linking failed when using Autotools build system targeting
    MinGW.
 *  [**Bug**] examples: Linking failed when using Autotools build system
    targeting MinGW.

 *  [**Change**] Support for mingw-std-threads when building for MinGW targets
    is now deprecated because this is not supported for GCC 11 or later (see
    <https://github.com/meganz/mingw-std-threads/issues/79>).

 *  Ping-pong sample loops sometimes stopped playing at the end of the loop.