AL/alext.h is not supported on html5 OpenAL implementation, just
replaced by the defines used in audio module

Only using raylib required extensions... it could be further reduced!

Some shader calculations are now pre-calculated because some math
functions doesn't exist in glsl 110.

... and switch from debug build to release

...on Android

Moved all external libraries used by raylib to external folder inside
raylib src. Makefile has already been update and also the different
includes in raylib modules.

Android build system has been simplified and now is included in src
folder, like other build systems.

Trying to find the best way to integrate Oculus support into raylib,
making it easy for the user...

Internal modelview and projection matrices can be replaced before
drawing.

To avoid additional dependencies

Moved AudioError enum inside audio.c

The only thing I did not change was the _g for globals. Is there any
other way we can mark globals?