Disable C11 atomics on OS X.

While the compiler on OS X sets the macros as if it supports C11
atomics, stdatomic.h is actually missing.

Change-Id: Ifecaf1c8df6390e6b994663adedc284d9b8130b7

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index d001d8a..8c38279 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -358,8 +358,12 @@
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && \
     !defined(__STDC_NO_ATOMICS__)
+/* OSX's atomic support is broken: the compiler sets the right macros but
+ * stdatomic.h is missing. */
+#if !defined(OPENSSL_APPLE)
 #define OPENSSL_C11_ATOMIC
 #endif
+#endif
 
 /* CRYPTO_REFCOUNT_MAX is the value at which the reference count saturates. */
 #define CRYPTO_REFCOUNT_MAX 0xffffffff