CMake 2.8 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, and our shipped SDL 2.0
headers when WITH_SDL_DYNLOAD=ON. This patch ensures that in both
cases the correct SDL headers are used.
Reviewers: sergey, campbellbarton
Projects: #bf_blender
Differential Revision: https://developer.blender.org/D1112
