configure: support --disable-sdl

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
configure: reindent the --disable-sdl related code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
configure: fail if SDL is unavailable and --enable-sdl is specified

Reviewed-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>