sdl: fix directfb configure check

The configure script nicely supports a preconfigured directfb-config
(using ac_cv_path_DIRECTFBCONFIG), except for when it checks the directfb
version where it simply uses whatever directfb-config is in the path,
breaking the build when this isn't available / recent enough.

Fix is to use the correct one instead. As SDL doesn't autoreconf cleanly,
also patch the generated configure script.

Finally move the ac_cv_path_DIRECTFBCONFIG setting under the directfb
conditional as it is only needed there.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>