Fixing bundled and static-link for macOS

[seivan]

If we can sorta squeeze in

  if(SDL_VIDEO_RENDER_METAL)
    find_library(METAL Metal)
    list(APPEND EXTRA_LIBS ${METAL})
    file(GLOB RENDER_SOURCES ${SDL2_SOURCE_DIR}/src/render/metal/*.m)
    set(SOURCE_FILES ${SOURCE_FILES} ${RENDER_SOURCES})
  endif()

in SDL's CMakeLists.txt before running this and set SDL_VIDEO_RENDER_METAL=1 for target_os == "darwin" then we can get both of those feature flags to work.

Obviously this belongs in upstream, but the process of getting pull requests to SDL is a bit tricky, I am hoping we can fix it in the crate by just injecting it into the string right before:

1509 //{Inject here}
1510 # iOS hack needed - http://code.google.com/p/ios-cmake/ ?
1511  if(SDL_VIDEO)
1512    if (IOS)