readd g_level_lockup symbol for backward compatibility when log4cxx is being used

@tfoote @wjwwood I am not sure if you have seen this:

* http://answers.ros.org/question/116436/missing-log4cxx-on-latest-ros_comm/
* https://github.com/ros-drivers/hokuyo_node/pull/8

Since I don't think that it is a good idea to pull the changes back out I would propose to readd the symbol in question (`g_level_lookup`) as done in this pull request. That backward compatible symbol can be removed for indigo again. Downstream packages will still need to explicitly state their dependency on log4cxx - I think that is reasonable since these packages are simply relying on a transitive dependency (which is gone now).