MDEV-33746 Supply missing override markings

Find and fix missing virtual override markings.  Updates cmake
maintainer flags to include -Wsuggest-override and
-Winconsistent-missing-override.

sql/handler.cc

@@ -162,7 +162,7 @@ public:
                         const char* sqlstate,
                         Sql_condition::enum_warning_level *level,
                         const char* msg,
-                        Sql_condition ** cond_hdl)
+                        Sql_condition ** cond_hdl) override
   {
     *cond_hdl= NULL;
     if (non_existing_table_error(sql_errno))