Add support for libopenmpt.

[CicTec]

Hi,

SDL2 currently supports libxmp and libmodplug as MODs decoders. I think it would be useful to add support for libopenmpt, first because it is the successor to libmodplug (which is no longer maintained), and second because it is used by OpenMPT, so those who want to use particular effects with this composition tracker could do so (in the absence of support in libxmp for example).