> I assume you are talking about users that do have the sources on their
> machine, so the only problem is that the source directory is not what
> is recorded in the binary. If so, the only problem I know of is that
> they need to type in the directory where they have the sources, only
> once per session, when Emacs prompts for it.
>
Yes, that is correct. The problem is how they should find out that.
Where is that information? I think that it should be there right when
they try to access the C sources from within help. And possibly
somewhere in the manual too.
If they didn't build Emacs from sources, but they have copied the sources,
they must have done so manually. Emacs has no way of knowing where those
sources are. Only the user would know.