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

[Paul Eggert]

On 11/18/2014 10:04 AM, Joseph Myers wrote:
> +	  DIAG_PUSH;
> +#if __GNUC_PREREQ (4, 7)
> +	  DIAG_IGNORE (-Wmaybe-uninitialized, 4.9);
> +#endif
>   	  if (cp[nhere - 1] > usrc[nhere -1])
>   	    {
>   	      cp += 2 * nhere;
> @@ -105,6 +117,7 @@ findidx (const int32_t *table,
>   	  /* This range matches the next characters.  Now find
>   	     the offset in the indirect table.  */
>   	  offset = usrc[nhere - 1] - cp[nhere - 1];
> +	  DIAG_POP;

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.

Also, the distinction between GCC version "4, 7" (which is checked) and 
version "4.9" (which is not) is bothersome. Inevitably the unchecked 
version numbers will become wrong.  If we're going to have a "4.9" at 
all, we should check it.

How about the attached (untested) patch instead?  This is simpler and 
less intrusive and should scale better.  It disables the diagnostic in 
some places that it doesn't absolutely need to, but it separates 
concerns better and overall I expect it would have a better 
cost-to-benefit ratio.
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