Improve autoconf detection of sdl(_image|_mixer) This commit makes VoR cross-compile for windows on linux using stock mingw-cross-env (make sdl-image sdl-mixer) with just ./configure --host=i686-pc-mingw32 && make This removes the m4 craziness and instead relies on pkg-config to tell us how to compile and link against SDL. pkg-config files for SDL_image and SDL_mixer are used if found, but are not required (they default to -lSDL_image and -lSDL_mixer respectively.) Also the check for a symbol in SDL_mixer was updated to try to check for vorbis support.