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

[Paul Eggert]

Joseph Myers wrote:
> This is the one such optimization-related warning I see on x86_64

Wow.  You're lucky.  I've run into quite a few such warnings with coreutils, 
Emacs, etc.  Perhaps it's because they enable more GCC warnings.

If this is the only warning we run into with glibc, then it doesn't matter much 
how it's addressed.  If we start running into this more, though, I expect the 
draft approach won't scale well.