Oracle Linux 7 sets __STDC_VERSION__ to 201112L, but doesn't define
__STDC_NO_ATOMICS__, so we try to include <stdatomic.h> without the
header present in the system. Since we are already detecting the header
in the autoconf, use the HAVE_STDATOMIC_H for more reliable detecting
whether <stdatomic.h> header is present.
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