spritesheetc should emit .d files for makefile inclusion

[Taywee]

.d files should be automatically emittable, in order to support better dependency management (forcing a recompile/reassembly in the case of a changed image input). spritesheetc can't automatically determine dependencies, so an argument will need to tell it which object file depends upon the images so it can properly emit its .d file (probably default to main.o or game.o), and another argument to tell it what inclusion filename to emit (with no default; without this argument, no file will be generated).