Plugins included into bin release cannot be
    installed on debug version of server

IF    the build process was split into separate "debug" and
      "optimized" builds
   AND
      the plugin files of the debug build got copied into
      "plugin/debug/"
(both is done for MySQL release builds starting from 5.5.3),
THEN these debug plugin files are to be included in the
     final binary package.

This change deals with the inclusion only,
the other parts are done in different changesets.