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

[Joseph Myers]

On Tue, 18 Nov 2014, Paul Eggert wrote:

> Joseph Myers wrote:
> > the relevant time to check is when 4.9 is no
> > longer supported
> 
> If that's the case, I'm having trouble seeing why it's worth our time to have
> that unchecked "4.9" into the code.  If the only time the "4.9" matters is
> when we bump the min GCC release, we can simply reverify all the
> version-dependent diagnostics when that happens.  That way, we don't need to

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.

-- 
Joseph S. Myers
joseph@codesourcery.com