CMake doesn't search /usr/include/SDL2, which is the include directory for SDL 2.x on Ubuntu Linux (and possibly others). This results in SDL 1.2 headers being found when WITH_SDL_DYNLOAD=OFF (or none, if SDL 1.2 is not installed). Our shipped SDL 2.0 headers are used when WITH_SDL_DYNLOAD=ON. This patch ensures that in both
cases the correct SDL headers are used.
Both CMake 2.8 and the latest 3.2.0-rc1 skip /usr/include/SDL2, so increasing the minimum required CMake version won't help.