add `pause_on_minimize` config option
prevent sdl2 from building if not building for this host
Merge pull request #2687 from kived/build-sdl2

prevent sdl2 from auto-building if not building for this host
do not override `pause_on_minimize` in kivy.__init__
set `pause_on_minimize` to 0 by default
remove leftover docstring
Merge pull request #2688 from kivy/pause_on_minimize

Add `pause_on_minimize` config option