Make's assumption that files it does not create do not change

Dear all,

David Allsopp (@dra27 on GitHub), in Cc, has submitted the following
pull request on the OCaml repository:

https://github.com/ocaml/ocaml/pull/11180

Countrary to what its title suggests, the PR is tightly GNU make related.

On the PR, David explains the problem in a very detailed way and
proposes a solution.

Both of us would be interested in a review by GNU make experts.

We are also wondering whether such situations wouldn't deserve a few
words of explanation in the manual (we believe this behaviour is
currently not explicitly documented).

Please, if you wish to react to this e-mail, we would appreciate if you
could do so on GitHub rather than here, unless it really has nothing to
do with the PR itself, of course.

Many thanks in advance for your attention,

Sébastien.