Only disable placement-new warning under gcc >= 6

Otherwise this would create unknown option warnings under g++ < 6.

include/pybind11/pybind11.h

@@ -82,12 +82,12 @@ protected:
 
         /* Store the capture object directly in the function record if there is enough space */
         if (sizeof(capture) <= sizeof(rec->data)) {
-#if defined(__GNUG__) && !defined(__clang__)
+#if defined(__GNUG__) && !defined(__clang__) && __GNUC__ >= 6
 #  pragma GCC diagnostic push
 #  pragma GCC diagnostic ignored "-Wplacement-new"
 #endif
             new ((capture *) &rec->data) capture { std::forward<Func>(f) };
-#if defined(__GNUG__) && !defined(__clang__)
+#if defined(__GNUG__) && !defined(__clang__) && __GNUC__ >= 6
 #  pragma GCC diagnostic pop
 #endif
             if (!std::is_trivially_destructible<Func>::value)