games-arcade/defendguin requires sdl-mixer to be built with mikmod support to enable playing the tracker music files. If mikmod support isn't available in sdl-mixer defendguin just dies. The attached patch just fires an error if the mikmod USE wasn't enabled when sdl-mixer was built.
Created attachment 84731 [details, diff] defendguin-sdlmixer_mikmod.patch
Fixed in CVS... thanks for the bug report and the patch!