bug#46837: Makefile dependencies for info files

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Sun, 28 Feb 2021 18:39:40 -0500
> 
> I suspect that the Makefile dependencies for info files could be
> incomplete as of 4825ea3c5d. The symptoms would be info related errors
> in parallel builds, and missing files in info/dir.
> I would appreciate it if someone would review it.

Is the fact that we now rebuild info/dir each time we run "make", even
if nothing else is rebuilt, part of this issue?