Log Message:
Update libopenmpt to 0.2.5705.

libopenmpt 0.2-beta15 (2015-10-31)

    [Change] openmpt123: SDL2 is now supported and preferred to
    SDL1 if available.
    [Bug] Emscripten support for older emscripten versions broke
    in -beta14. These are now supported again when using make
    CONFIG=emscripten-old.
    [Bug] Fixed crashes in MED loader.
    Playback improvements and loader fixes for MOD, MT2 and MED.