This moves the SDL bits for window ID from the baum driver to SDL, as
well as fixing the build for non-X11.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-id: 20161221003806.22412-3-samuel.thibault@ens-lyon.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>