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

[Joseph Myers]

I should add: I do *not* think we should have any sort of -Werror or 
diagnostic control policy that involves this level of analysis for each 
patch disabling warnings; it should be sufficient that either the warning 
appears to be a false positive without an obvious easy and safe fix or 
there is some other reason it's hard or inappropriate to fix (e.g. tests 
of deprecated interfaces, _FORTIFY_SOURCE checks that also generate 
compile-time warnings, etc.).

-- 
Joseph S. Myers
joseph@codesourcery.com