SDL: Refactor package and fix Darwin cross-builds.

First of all, this needs two upstream patches for supporting OS X 10.9,
also the cross-gcc doesn't accept -fpascal-strings, so drop the flag.

And except putting all configure flags into a multiline string, we're
now using a list, which is easier when it comes to handling optional
components.

As X isn't used on recent Mac OS X versions and Windows, I'm temporarily
using --without-x for cross-builds until we have a better way to check
for those things.

Also, don't add audiofile to buildInputs if we're cross-compiling for
MinGW.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>