sdl: add support for top-level parallel make

To be sure that host-autoconf dependency is already built move the
call to autogen.sh from SDL_POST_PATCH_HOOKS to SDL_PRE_CONFIGURE_HOOKS.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>