scons: Add warning for missing declarations

This patch enables warnings for missing declarations. To avoid issues
with SWIG-generated code, the warning is only applied to non-SWIG
code.
diff --git a/src/sim/sim_object.hh b/src/sim/sim_object.hh
index 6424f63..1cb06d7 100644
--- a/src/sim/sim_object.hh
+++ b/src/sim/sim_object.hh
@@ -176,4 +176,8 @@
     static SimObject *find(const char *name);
 };
 
+#ifdef DEBUG
+void debugObjectBreak(const char *objs);
+#endif
+
 #endif // __SIM_OBJECT_HH__