Add emulated Serial MIDI synthesizer support

[mooinglemur]

This PR adds serial MIDI support

• The "wavetable" support is emulated by FluidSynth with a loadable SoundFont. I have not found an unencumbered General MIDI soundfont that we could distribute legally beyond a reasonable doubt, so it's up to users to obtain and load their own.
• FluidSynth can also hook up the default system MIDI IN, and x16emu can tie it to the serial input of the first emulated UART.
• FluidSynth's internal MIDI synth audio is mixed into the rest of x16emu's audio, alongside VERA and FM.
• Unfortunately, FluidSynth does not present the operating system's MIDI OUT, so the internal synth is currently tied to both UARTs' outputs. With additional work we could use the native MIDI libraries on each operating system to plumb the first UART to a hardware MIDI out in the future.
• As FluidSynth runs in its own threads, and MIDI input is event driven with a threaded callback into x16emu code, x16emu needs to handle the buffers in a thread-safe way, so this is the beginning of thread code in x16emu.
• fluidsynth is now a build dependency for x16emu but not a strict runtime dependency, as we are using dynamic library loading.

[Fulgen301]

Suggested change

	fprintf(stderr, "Warning: Serial MIDI card address must be in the range of 9F60-9FF0\n");
	fprintf(stderr, "Warning: Serial MIDI card address must be in the range of $9F60-$9FF0\n");

$9FF0 is in the middle of IO7, is that a typo? Might also be worth mentioning that that is IO3-IO6/IO7.

[mooinglemur]

The way -midicard's argument is parst does not handle $, and such the error could be confusing when $ is given, unless we handle $.

Also, the MIDI (and serial) cards only use half of an I/O range, 16 registers for two UARTs, so the IO range really doesn't apply as much as the second-to-least-significant nibble of the address.

[Fulgen301]

Who is this warning geared towards? People developing MIDI software or regular users that look at their program output? If I read that message on a terminal, I'd have no idea what LCR is, or whom I should report that to.

[mooinglemur]

This is meant for 6502 programmers trying to write software to talk to the MIDI card, and it means they got something wrong. I figure this is better than no message and no MIDI sound (which is what they'd get on hardware) or it works despite the wrong settings (which would lull the programmer into the false notion that they got it right)