on some older perls, assigning undef generates a warning

e.g. see
http://www.cpantesters.org/cpan/report/3fb87c02-a74d-11e3-9ba1-e54111bde73e
Allow dump_config to include all the content brought in by superclasses

AfterBuild recomposes ConfigDumper, wiping out the method modifications
to dump_config done by [MakeMaker]