Check for sdl-config before calling it

Check whether sdl-config is available before calling it, otherwise
./configure triggers a warning:
    ./configure: 957: sdl-config: not found

If neither the .pc file not sdl-config are present, disable SDL support.

Signed-off-by: Loïc Minier <lool@dooz.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
