minor: clang may use an older than GCC 5 libstdc++

Author: acmorrow

src/mongocxx/instance.cpp

@@ -69,7 +69,7 @@ typename std::aligned_storage<sizeof(instance), alignof(instance)>::type sentine
 std::atomic<instance *> current_instance{nullptr};
 static_assert(std::is_standard_layout<decltype(current_instance)>::value,
               "Must be standard layout");
-#if (!defined(__GNUC__) || defined(__clang__)) || (__GNUC__ >= 5)
+#if (!defined(__GNUC__) || (defined(__clang__) && !defined(__GLIBCXX__))) || (__GNUC__ >= 5)
 static_assert(std::is_trivially_constructible<decltype(current_instance)>::value,
               "Must be trivially constructible");
 #endif