Build without ALSA MIDI by default

[David]

Trying to build with the default options on some Linux distros will (correctly) detect ALSA - but due to API changes or whatnot, compile fails with a bunch of errors like these:

a_midi.c:191: error: ‘SND_RAWMIDI_OPEN_INPUT’ undeclared (first use in this function)
[...]
a_midi.c:192: error: ‘SND_RAWMIDI_OPEN_NONBLOCK’ undeclared (first use in this function)
[...]

I'd suggest just disabling or removing this code, as it's just a development tool (music authoring) that I probably won't even use again.