urg: make sdl support optional

Only enable SDL support if SDL and SDL_net are already enabled in the
configuration.

[Peter: drop SDL comment]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>