Support external GLFW usage
Renamed some flags for consistency

Updated to latest GLFW library and working on Wayland support, still
looking how to implement it on raylib because it just exposes
PLATFORM_DESKTOP and defaults to X11 windowing system on Linux...

OpenAL Soft backend is still available in audio module, I'm thinking if
exposing it for building in some way or just left it there for advance
users to switch to it manually in case of necessity...

They were disabled because they failed to build,
but this patch set fixes the build on Linux and macOS.

This doesn't apply to the AppVeyor build on Windows yet;
it currently fails at linking with OpenAL.

libEGL and libGLESv2 have been renamed on latest Raspbian Stretch
version (also included on latest Jessie upgrade).

It seems to be done to avoid conflicts between official Mesa libs and
Broadcomm provided ones.

Not tested, could have errors

Could correct possible issues on OpenGL 1.1

- Review RPI compilation (core_drop_files not supported)
- Review ImageFormat(), some issues
- GetTextureData() reviewed for RPI

...and reviewed some details

Some people might find this handly

...to avoid issue on Arch Linux

We need to grab the OS name before determining what the path should be.