Since switching to git, I'm increasingly of the mind that clean) belongs to the source control (`git clean') rather than the build. After all, the source control knows what files belong to a bare checkout of the project and which have shown up since you got your grubby little paws on the code!

git clean has options to only clean ignored files, which generally does what you want. I could see there being weird situations where a file is .PRECIOUS (in make's terms) or where you have a file that's ignored but not a product of the build (a developer-specific config file?) where this doesn't work, but I haven't run into that yet, and it's freeing to not have to maintain a free target by hand.