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

[Paul Eggert]

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.