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

[Joseph Myers]

On Tue, 9 Dec 2014, Roland McGrath wrote:

> That's OK by me.  But it would be better if the explanations about how GCC
> limitations influenced the macro API choices were in comments in the header
> itself.

I've committed the patch with this comment.

/* The macros to control diagnostics are structured like this, rather
   than a single macro that both pushes and pops diagnostic state and
   takes the affected code as an argument, because the GCC pragmas
   work by disabling the diagnostic for a range of source locations
   and do not work when all the pragmas and the affected code are in a
   single macro expansion.  */

-- 
Joseph S. Myers
joseph@codesourcery.com