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.

src/sim/debug.hh

@@ -53,6 +53,8 @@ void takeCheckpoint(Tick when);
  */
 void eventqDump();
 
+void py_interact();
+
 int getRemoteGDBPort();
 // Remote gdb base port.  0 disables remote gdb.
 void setRemoteGDBPort(int port);