sdl: remove dga support in no MMU

DGA support use fork. This prevent executables to link with libSDL.so.

Fixes:
  http://sysmic.org/~jezz/results/5e50c243 (package sdl_mixer)
  http://sysmic.org/~jezz/results/734afef8 (package sdl_net)
  http://sysmic.org/~jezz/results/e2310eb9 (package sdl_image)
  http://sysmic.org/~jezz/results/e2cdcc9a

 (package sdl_sound)

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>