[AD] macro namespaces

[Peter Wang <novalazy@xxxxxxxxxx>]

Hi,

Milan's latest commit adds a platform configuration macro ALLEGRO_OGL. I
wondered why it was called that instead of ALLEGRO_OPENGL, but of course
ALLEGRO_OPENGL is already the name of a display flag.  Similarly with
ALLEGRO_DIRECT3D and ALLEGRO_D3D.  I think we need to start
distinguishin the configuration macros from other macros, e.g.
ALLEGRO_CFG_OPENGL.  We had the same confusions in Allegro 4.

Peter