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.


plugin/Makefile.am:
  The "install" rule must not assume that "debug" files are
  present, this depends on the preceding steps in the build.
  
  If they are present, we cannot copy them by simply using
  "libtool install" (as we could do during an ordinary build)
  because they are already kept in "plugin/debug/".
  
  When doing the copy to the destination hierarchy, we must
  ensure that symlinks are not expanded but rather copied
  as symlinks.
  "cp -d" is specific to GNU, the portable way would use
  "cp -P", but that is not fully specified with recursion.
  So we fall back on "tar c | tar x", which is known to
  keep symlinks unchanged (and un-expanded).
  
  Using "$(TAR)" is just a precaution in case of weird path
  settings or other portability issues.