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

[Joseph Myers]

On Fri, 21 Nov 2014, Paul Eggert wrote:

> Joseph Myers wrote:
> > The point is it's something you can grep for so that only a subset of the
> > diagnostic disabling needs checking each time the minimum GCC version
> > increases.
> 
> This worries me, because it implies these macros will be so commonly used that
> the cost in adding (and more importantly, having developers read) GCC version
> numbers every time the macros are used is worth the benefit of checking only a
> subset of the uses once in a blue moon.  If these macros are rarely used, the
> version numbers won't help much; and if the macros are used often I expect we
> will have so many other problems that the version numbers won't help much.

Even with a few uses, it's still helpful to be able to check for new GCC 
versions incrementally rather than requiring all the checks for all pragma 
uses to be done at once, or the state of which have been checked to be 
tracked externally - as soon as there are pragmas for warnings seen on 
multiple architectures, it may not be convenient for the person increasing 
the required GCC version to check whether all the warnings are still 
applicable, and so the version numbers mean the state of the partial 
transition is visible in the source tree.

-- 
Joseph S. Myers
joseph@codesourcery.com