bug#21953: Eliminate warnings in the emacs-25 release branch

[Glenn Morris <rgm@gnu.org>]

Eli Zaretskii wrote:

> I agree that we cannot silence or solve all of them, but we should be
> able to do that with most of them.

I'm no fan of compilation warnings and have done my bit to remove them
over the years. There was a brief glorious period a long time ago when
there were none, but it did not last, and I doubt it will return.

Actually really removing all warnings will require changes of a scale
and nature that are incompatible (IMO) with a release branch, and
break backwards compatibility all over the place. There may be a
handful of issues that are "real" and can be fixed simply, but most of
these are about relatively deep changes in Emacs infrastructure (eg
prep for lexical binding) for which the codebase hasn't been entirely
updated. Slapping with-no-warnings all over the place would be
pointless at best, frankly harmful since it just covers the issue up
without dealing with it (might as well just set byte-compile-warnings
= nil in the Makefile for all the good it would do).

In any case, .elc files are pre-built in release tarfiles, so people
building releases don't even see these warnings. So making it a release
goal doesn't make sense to me.