--- configure.orig	2019-03-20 16:57:56.000000000 +1100
+++ configure	2019-04-02 02:36:21.000000000 +1100
@@ -18020,10 +18020,27 @@
    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <inttypes.h>
+
+#if !defined(__has_feature)
+#define __has_feature(x) 0
+#endif
+
+#if !defined(__has_extension)
+#define __has_extension(x) __has_feature(x)
+#endif
+
+#if __has_extension(c_atomic) || __has_extension(cxx_atomic)
+#define __CLANG_ATOMICS
+#endif
+
 int
 main ()
 {
+#ifdef __CLANG_ATOMICS
+int32_t val = 0; __c11_atomic_fetch_add(&val, 1, __ATOMIC_RELAXED);
+#else
 int32_t val = 0; __atomic_fetch_add(&val, 1, __ATOMIC_RELAXED);
+#endif
 
   ;
   return 0;
--- lib/isc/unix/include/isc/stdatomic.h.orig	2019-03-20 16:57:56.000000000 +1100
+++ lib/isc/unix/include/isc/stdatomic.h	2019-04-02 03:33:45.000000000 +1100
@@ -32,7 +32,7 @@
 #endif
 
 #if !defined(__CLANG_ATOMICS) && !defined(__GNUC_ATOMICS)
-#if __has_extension(c_atomic) || __has_extension(cxx_atomic)
+#if defined(HAVE___ATOMIC) && (__has_extension(c_atomic) || __has_extension(cxx_atomic))
 #define __CLANG_ATOMICS
 #elif __GNUC_PREREQ__(4, 7)
 #define __GNUC_ATOMICS