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
