Improve autoconf detection of sdl(_image|_mixer)

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.