bug#29462: 24.4; eval-when-compile won't mute warning as says in info doc

[Eli Zaretskii]

> From: Emanuel Berg <moasen@zoho.com>
> Date: Mon, 27 Nov 2017 03:49:42 +0100
> 
> It sure send mixed signals and as it stands one
> gets the impression what you should do is find
> ways of silencing warnings, instead of
> improving the code so there is nothing to warn
> about to begin with.

In many real-life situations, there's no cleaner way of avoiding such
warnings, except to require the package which defines the features you
are warned about.  There's nothing wrong about doing that, which is
why you will find such uses all over the Emacs sources.