Allow compiling the library with support for automatic screen capture
(KEY_F12)

In case mesh loading fails, a cube is generated instead!

Add more key definitions

Seems tcc doesn't provide __sync_*. See #435.

Reported in #435, tcc generates out-of-line definitions for inline
symbols, something it shouldn't.  This fix punishes tcc for that
by making applications it compiles (slightly) larger...

Added raylib version to config

Refactor all #define SUPPORT_* into a config.h

Added GetCollisionRayModel

I would have liked config.h to be selected by include dir configuration,
but this way is less intrusive.

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).

Now PLATFORM checks only used on core and utils modules

Renamed SaveImageAs() to ExportImage() for consistency on what actually happens with data.

Removed OpenAL and GLFW3 dependencies on building.
Reviewed platforms and flags

Not sure if math is ok... just left a commented piece of code that uses pre-multiplied alpha.

Fix potential bugs from static analysis

Note that only some lib versions have been updated

Reviewed for PLATFORM_ANDROID and PLATFORM_UWP

Allow use of main instead of android_main