Built-in dependency that says if the current build is processing any changes

Firstly, thanks for making dune. It roolz!

Desired Behavior

I'm currently depending on universe to cause a build-info file to be re-generated on each dune build.

See more background here: https://discuss.ocaml.org/t/rebuilding-a-build-info-module-every-time-an-exe-is-produced/4954/3

This works, it definitely causes a rebuild if stuff anywhere else in the repo change, which is needed for a proper build-info. What it also does is rebuild even if nothing changes, which isn't great if you have several hundred .exe files to build. That is, a make followed by make does a lot of extra work, rebuilding binaries that didn't change.

Example

Something like, say, work-to-do if dune detects any source files have changed instead of universe would be grand.