One of the aims of butter is to build only what needs building. Presently every source file is compared with the object that it should result in and the final binary for “up to dateness” and things built as required. I’m planning on also building if the Instructions file in the directory is newer or if the Recipes file it’s using is newer, but the question is how far do you go with building for updated files?
I also need to get round to looking at adding the auto-dependancy stuff, but I’m just trying to fixup the command code at present to make it far more flexible and less “involved” that what’s there at present.