[AD] AL_ prefix clashes with OpenAL

[Elias Pschernig <elias@xxxxxxxxxx>]

One point we are currently discussing in the meeting is the clash of the
AL_ prefix for Allegro 5 with OpenAL. See for example here (a header
from openal):
http://www.koders.com/c/fidADF1216C9DD667D79D118F03D4D441F5689606EA.aspx?s=al.h

The prefix for functions, al_, is not used by OpenAL, so no problem with
that - but there's a high probability a type name or constant name will
clash with one used by A5 (and they might add further stuff, not caring
about us).

So, our options are:

* keep AL_ and al_ and try to not clash somehow (we *do* want OpenAL to
be used together with Allegro, most likely it even will be one of our
sound backends)

* keep al_, but use A_ ALG_ ALX_(Allegro eXtended) A5_ ALLEGRO_ or
something instead of AL_

* replace both al_ and AL_ to something like above

One idea also was to change the name of Allegro itself to e.g.
AllegroPro, then use ap_ and AP_ prefixes.

We couldn't really decide this, so asking here instead. (Myself, I'm
leaning to a5_ and A5_ currently.)

-- 
Elias Pschernig <elias@xxxxxxxxxx>