Support using system libraries

[theoparis]

I have https://github.com/libsdl-org/sdl2-compat installed on my system but KeyCodes.hpp uses a hard coded third_party directory instead of <SDL2/SDL.h>...
Same with zlib/zlib-ng, GLESv1 libraries from ANGLE, raknet, etc.
I think the correct solution would involve modifying CMakeLists.txt to use PkgConfig and modifying source files to use the correct SDL2 header path

[iProgramMC]

This actually already happens. Well, except for Android, where we have to use a fork of SDL2 that works on there. The only place it never does is in KeyCodes.hpp.

I think simply cloning recursively and compiling will do the trick. I don't see why the SDL2-Android key code definitions would differ from your system-installed library's.

The zlib library is vendored to fix a compilation error on windows. I don't remember why we vendor RakNet but it is for a good reason. Other than changing the protocol version from 6 to 4 it might work with standard raknet as a submodule.