Now compiles with latest cmake

cmake's "FindPackage" system recently changed SDL_mixer and SDL_image
from reporting their libraries as "SDLMIXER_LIBRARY" and
"SDLIMAGE_LIBRARY" to "SDL_MIXER_LIBRARY" and "SDL_IMAGE_LIBRARY",
respectively.

I prefer the new names, but I do wish they hadn't made this change;
I dislike that it forces build scripts to either be broken for new
version of cmake or be broken for old versions of cmake;  no
compatibility seems to be being provided to produce things compatible
with both.