funopen is only needed on Android, so declare it
only there. This fixes following MinGW build failure:

utils.c:50:7: error: conflicting types for ‘funopen’
 FILE *funopen(const void *cookie, int (*readfn)(void *, char *, int),
       ^~~~~~~
In file included from utils.c:45:0:
/usr/include/stdio.h:586:7: note: previous declaration of ‘funopen’ was here
 FILE *funopen (const void *__cookie,
       ^~~~~~~

REVIEWED: ExportImage()
REVIEWED: ExportMesh()
ADDED: ExportWave()
REMOVED: Internal funcs: SavePNG(), SaveBMP()

NOTE: These changes break the API (parameters order)

Did this ever work? Surely, doesn't look like it...

Not sure if this ever worked, but now it at least compiles.

That way, a user needs only to touch a single file to configure what
features raylib is built with.
Include guards are left out intentionally, because config.h should only
be included in source files, not headers.

Later on, config.h can also define the raylib version (#461).

I think is clearer this way...

Trace log messages could be configured with this function to select wich
ones are shown

Let the user choose if using rres external library

...to avoid possible conflicting symbols

Replaced old GetExtension() function
Make IsFileExtension() public to the API

Working in modules configuration flags...

- Renamed WritePNG() to SavePNG() for consistency with other file
loading functions
- Renamed WriteBitmap() to SaveBMP() for consistency with other file
loading functions
- Redesigned SaveBMP() to use stb_image_write

Start removing old rRES functions.

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.

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

Useful to save small data files (configuration and so)
For bigger files, external data storage should be used (SDCard)

Corrected some bugs on the way...
Automatically convert textures to POT on RPI and WEB

Added PLATFORM_WEB support (emscripten-webgl)
[audio] Added LoadSoundFromWave()
[textures] Added LoadTextureFromImage() to replace CreateTexture()
Some TraceLogs edited...