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

[Roland McGrath]

> My kneejerk reaction is that attempting to minimize the scope of 
> disabled warnings will make code harder to read and maintain, and the 
> costs of minimization may outweigh the benefits.

I strongly disagree.  We should only ever disable warnings with the
tightest possible scope around where we know for sure we need to.