bug#25791: atomics on macOS

[Matt Wette]

I saw this too and gave up trying to fix it.  I think this needs to be worked bottom up: I believe the C99 standard is specific about the type qualifier _Atomic being present for the first argument.  The workaround for now IMO, is to change atomics-internal.h from this:

#ifdef HAVE_STDATOMIC_H

to this:

#if (defined HAVE_STDATOMIC_H) && (!defined __clang__)

By the way, gcc is forgiving here.

Matt