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

[Paul Eggert]

Joseph Myers wrote:

> I'm doubtful about hiding warnings in ways that rely on the compiler not
> being smart enough to see through them ...
> An inline wrapper would not be appropriate; these tests are specifically
> using arguments with side-effects, and arguments with unusual types

Fair enough on both points, but this underscores the suggestion that for test 
cases let's do just put a simple pragma at the start of the file.  To grep it 
later we can use a stylized comment, like this:

/* (GCC 4.9) gcc -Werror -Wformat fails in C99 mode because ... */
#pragma GCC diagnostic ignored "-Wformat"

as it's not worth our time (or our readers' time) to get fancier on test cases.