Add macros for diagnostic control, use them in locale/weightwc.h

[Joseph Myers]

On Tue, 18 Nov 2014, Paul Eggert wrote:

> > Disabling for the relevant block of source lines, on all architectures and
> > for all GCC versions that support the relevant -W option, is a pragmatic
> > choice
> 
> Yes, and I guess my point is that the pragmatic choice we've taken in Gnulib
> is to disable diagnostics at the top level, as GCC has too many bugs in this
> area for us to go on wild goose chases trying to fine-tune diagnostics in
> smaller areas.  I'm skeptical whether it'll be worthwhile for glibc to chase
> those geese either, at least in the near future.

This is the one such optimization-related warning I see on x86_64 
(building glibc, rather than the testsuite - but in the testsuite we can 
be quite free with using dumb code, redundant initialization etc. to avoid 
warnings, without being at all concerned about resulting inefficiencies) 
with 4.9.  I think there are only a handful I've seen on other 
architectures.  (There may of course be some currently disabled with -Wno- 
options for whole files in the makefiles.)

-- 
Joseph S. Myers
joseph@codesourcery.com