Add built-in FMMIDI support to Autotools

[fdelapena]

As requested by Ghabry (#752), an opted-out by default FMMIDI option for Autotools.

The --enable-* option differs from --with-* because this is a built-in feature, not an external package. That's why I suggest to name it --enable-fmmidi without the "builtin", as it feels redundant.

This will need #ifdef HAVE_FMMIDI guards (don't forget #include "system.h") where used.

CMake users: Just declare -DHAVE_FMMIDI as usual.

[Ghabry]

Iirc someone also suggested a way to fallback to use sdl midi by default and fallback to fmmidi when it fails. But that needs another audio-sdl code change. Not sure if its currently worth it.

[carstene1ns]

I also had some commit for it prepared: carstene1ns@94fa946
We talked about allowing fallback mode for fmmidi if sdl_mixer does not play midi. WANT_FMMIDI can be 1 or 2.
Also i really dislike having empty objects compiled and linked in, so i disabled them if unneeded.

[carstene1ns]

And a friendly reminder to guard the fmmidi sources properly.
Currently it is bloat and will stay bloat, even if someone -disable[s]-fmmidi.
(I know you also wrote it above, but this is important IMHO)

[Ghabry]

👍 for your fmmidi want (fallback) and guarding.

[fdelapena]

carstene1ns' autotools branch is definitely better, could I discard this PR?

[Ghabry]

Okay. I will cherry pick carstene1ns commit into my branch instead.