Fix atomic builtins detection

[Ralph Boehme]

On Mon, Apr 25, 2016 at 11:15:42AM -0700, Jeremy Allison wrote:
> On Mon, Apr 25, 2016 at 04:24:28PM +0200, Jeremie Courreges-Anglas wrote:
> > 
> > Hi,
> > 
> > lib/replace/wscript has this bunch of code:
> > 
> >     # Check for atomic builtins. */
> >     conf.CHECK_CODE('''
> >                     int main(void) {
> >                         int i;
> >                         (void)__sync_fetch_and_add(&i, 1);
> >                         return 0;
> >                     }
> >                     ''',
> >                     'HAVE___SYNC_FETCH_AND_ADD',
> >                     msg='Checking for __sync_fetch_and_add compiler builtin')
> > 
> > 
> > It is incorrect, since CHECK_CODE already wraps the code fragment in
> > main().  Here's what gets compiled:
> > 
> > 
> > [includes]
> >  int main(void) { 
> >                     int main(void) {
> >                         int i;
> >                         (void)__sync_fetch_and_add(&i, 1);
> >                         return 0;
> >                     }
> >                     ; return 0; }
> > 
> > 
> > Now the funny part is when you run this where __sync_fetch_and_add is
> > actually not available.  The inner main() function isn't called
> > anywhere, and at cc -O2 it is simply optimized out, thus the compile
> > test wrongly succeeds.  This can be seen for example on OpenBSD/hppa
> > (gcc-4.2.1).
> > 
> > Please find attached a patch to fix this issue for __sync_fetch_and_add
> > and atomic_add_32.
> 
> Looks good to me. Can I get a second Team reviewer ?

lgtm. Can you push? Thanks!

Cheerio!
-slow