new field in `declare_dependency()` to depend on a `custom_target()` being run but without passing its `output`

[1ace]

The idea is that we want to say "that other target needs to be run (or skipped, normal run/skip rules apply here), but ignore its output:".

This is so that it doesn't get mixed in with the rest of the files of whatever uses this dep.

---

Note: on IRC I had said this was similar to "order-only" prerequisites in makefiles (https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html), but I was wrong, as we do want to keep track of the modification time of the output: (make doesn't) so that we know when the custom_target() needs to trigger a rerun of all the targets that depend on it; we just want to not mix in the output file.

[dcbaker]

We generally have these kind of rules as depends in other places.

Another place this might be required is for a library that will need to dynamically load a library, but doesn't link to it.