Add macros for diagnostic control, use for scanf %a tests

[Paul Eggert]

Joseph Myers wrote:
> +  /* GCC in C99 mode treats %a as the C99 format expecting float *,
> +     but glibc with _GNU_SOURCE treats %as as the GNU allocation
> +     extension, so resulting in "warning: format '%a' expects argument
> +     of type 'float *', but argument 3 has type 'char **'".  This
> +     applies to the other %as, %aS and %a[] formats below as well.  */
> +  DIAG_PUSH_NEEDS_COMMENT;
> +  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
>     if (sscanf (" 1.25s x", "%as%2c", &sp, c) != 2)
>       FAIL ();
>     else
> @@ -32,10 +40,14 @@ main (void)
>         memset (sp, 'x', sizeof "1.25s");
>         free (sp);
>       }
> +  DIAG_POP_NEEDS_COMMENT;


This sort of thing looks intrusive.  Instead, how about defining a macro 
'C99_sscanf' that does the push and pop and commentary, and then simply 
replacing warned-about uses of sscanf with C99_scanf?  That'd be a lot easier to 
read, and it's more along the lines of Roland's original suggestion.