[FYI 0/2] Fix -Wformat-nonliteral

[Tom Tromey <tom at tromey dot com>]

Simon pointed out today that my earlier patch to remove some custom
rules from the Makefile had broken his build.

We tracked this difference down to a bug in warning.m4:
-Wformat-nonliteral was not being enabled for builds with gcc.  This
is why my build and the builders did not warn -- in fact the original
patch was incorrect.

This series fixes the problem using the diagnostic suppression code.
This seems a bit better because now the warning avoidance is more
targeted.

The first patch here is from Simon (I wrote the commit text and the
ChangeLog entry, in case they aren't up to par); but I'm committing it
on his behalf as he is traveling.

I'm checking this in.

Tom