Support the ability to re-run build scripts if and only if the crate is being compiled for other reasons

Describe the problem you are trying to solve

We embed a bit of build-time metadata (e.g. git commit hash, wall-clock build time) inside our binaries. We specifically only would like to change this metadata on re-build, we don't want to force a rebuild because the metadata has changed. For example, checking out a commit that has only changed external documentation shouldn't retrigger a build, none of the source has changed. Also, when running tests, modifying the tests shouldn't necessarily retrigger a build of the library that the tests are running against.

Describe the solution you'd like
Perhaps a cargo:rerun-on-compile=true build.rs directive?

Start with Cargo's build-script rerun behavior and the proposed cargo:rerun-on-compile=true directive. Compare how rebuild decisions should behave when only external documentation or tests change. Done means the directive's semantics and affected build cases are specified clearly enough to guide implementation and validation.