Detect and use SDL2 with help of this Dolphin CMake module:
https://github.com/dolphin-emu/dolphin/blob/master/CMakeTests/FindSDL2.cmake
Look for SDL_gamecontroller.h instead of SDL.h to prevent CMake from
using SDL1 include dirs
Update to SDL2
Update native submodule
Merge branch 'master' into sdl2

Conflicts:
	native
Merge branch 'sdl2' of github.com:PGGB/ppsspp into PGGB-sdl2