sdl_mixer: Make linking against libmad an explicit choice

The MAD library is licensed under GPL, so even applications linking to
it dynamically must be under a GPL-compatible license. Therefore, on
systems where both GPL and non-GPL licensed applications want to use
SDL_mixer, SDL_mixer cannot be linked to libmad.
opendingux: Do not link SDL against tslib

Typically systems with tslib want to use it in SDL too, but we don't.
sdl_image: Bump version to 1.2.12
Revert "SDL_image: fix runtime libjpeg failure"

This reverts commit 883f066797ae0e7a017fac8e275f6e58659db760.

Bug in configure script was fixed upstream in SDL_image 1.2.8.