Minor cmake issue

As a quick warning to anyone who might be looking at the VectorStorm codebase, as of cmake version 2.8.10 (or thereabouts), the variables describing where to find SDL_mixer and SDL_image have changed from SDLMIXER_* and SDLIMAGE_* to SDL_MIXER_* and SDL_IMAGE_*, respectively.  Which caused me a good fifteen minutes of confusion before I was able to work out where my linker errors were coming from.

I like the new variable names a lot more than the old ones; smashing the words together always seemed like a strange choice.  But this sort of breaking change to the exposed interface is really disappointing.  Now I need to figure out what I’m going to do with VectorStorm’s build configuration to allow people (ie: me) to do builds with either new or old builds of cmake.  Or else I have to just mandate that everyone update.  Maybe I need some build conditionals to check which variables have been set, and work around the thing.

Will post here when I figure out a reasonable solution.  For now, if you’re using cmake 2.8.10, you’ll need to change the names of those variables in the CMakelists.txt files.