Add support for reference counting using C11 atomics

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>

GH: #1500
diff --git a/crypto/engine/eng_list.c b/crypto/engine/eng_list.c
index 934389f..d8eb076 100644
--- a/crypto/engine/eng_list.c
+++ b/crypto/engine/eng_list.c
@@ -349,6 +349,6 @@
         ENGINEerr(ENGINE_F_ENGINE_UP_REF, ERR_R_PASSED_NULL_PARAMETER);
         return 0;
     }
-    CRYPTO_atomic_add(&e->struct_ref, 1, &i, global_engine_lock);
+    CRYPTO_UP_REF(&e->struct_ref, &i, global_engine_lock);
     return 1;
 }