[AD] support for mod files

[Matthew Leverton <meffer@xxxxxxxxxx>]

I've created an addon designed to be part of acodec that adds support
for MOD, IT, XM, and S3M files via libdumb (external dependency). I
can commit it if there are no objections.

I did not implement loading into an ALLEGRO_SAMPLE, as I don't think
it's very useful since there's no guarantee that the mod file loops
from end to start.

The add-on currently creates "liballegro_mods.so":

# pkg-config allegro_mods-4.9 --libs
-L/usr/local/lib -lallegro_mods -lallegro_audio -lallegro

A few of the functions include:

al_init_mods_addon();
al_load_mod_audio_stream();
al_load_xm_audio_stream_f();

I didn't want to call it "allegro_dumb," as I think the add-on name
should reflect its functionality, not the external dependency. (The
dependency should be able to be changed without having to rename the
library.)

However, I realize "mods" is a generic name. Perhaps "audio_mod" or
"audio_mods" or something entirely different would be better. I'm open
to suggestions.

--
Matthew Leverton