...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?

This is a first step toward a bigger project. Some modules could be
ported to header-only to be used as standalone.

Checking some files to be converted to header-only

Tint color could be applied to colDiffuse... but what's the best way?
Replace it? Multiply by? A point to think about...

Functions renamed to improve library consistency