Provide a function to register a post build routine to do the warnings. Do that
way so that the user can chose when to add the callback to the build (i.e.
allow the user to assert a desired order for the post build callbacks to be
called).
By running waf with option -v, waf was complaining about the update_submodule
tasks having the same identifier. That happened because the default uid()
method depended on the class name, inputs and outputs. Since this type of task
doesn't have input nor output file, we need to override uid() so that it uses
the submodule path for the task identifier.
