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

Add orthographic 3d rendering mode

This is already the order that is used for Android. It doesn't appear to
make a difference on desktop but on web using the timer before it's been
initialized (by glfwInit, inside InitGraphicsDevice) causes the a long
(and variable but often several seconds) sleep between the first and
second frame.

Fixes: 468309d ("Early-exit InitWindow if InitGraphicsDevice fails")

Changes motivated by commentary in pull request 513

Changes made based on commentary in pull request 513

Fix an infinite loop in ALSA backend of mini_al.

...PLATFORM_ANDROID on Windows, using MinGW-w64 (x86) provided Make (GCC
7.2)

Fix a buffer overflow in the OSS/BSD backend of mini_al.