cmake: re-enable -Werror in the maintainer mode

now we can afford it. Fix -Werror errors. Note:
* old gcc is bad at detecting uninit variables, disable it.
* time_t is int or long, cast it for printf's

storage/mroonga/lib/mrn_context_pool.cpp

@@ -77,7 +77,7 @@ namespace mrn {
     }
 
   private:
-    static const unsigned int CLEAR_THREATHOLD_IN_SECONDS = 60 * 5;
+    static const int CLEAR_THREATHOLD_IN_SECONDS = 60 * 5;
 
     mysql_mutex_t *mutex_;
     LIST *pool_;