Workaround faulty stdatomic.h header detection on Oracle Linux 7
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.
Closes #4270 (closed)