* lib/am/distdir.am (top_distdir, distdir): Define these

* lib/am/distdir.am (top_distdir, distdir): Define these
variables only in top level Makefile.  Redefine $(top_distdir)
as $(distdir).
(distdir): Fix passing of $(top_distdir) to sub-makes.
* automake.in (handle_dist): Do not define the DISTDIR and
TOP_DISTDIR transformations.  We don't need them anymore.
* automake.texi (Dist): Reorder the dist-hook section, and move
the DIST_SUBDIRS paragraph earlier.  Document distdir and
top_distdir.
* tests/subpkg.test: Update to check for $(distdir) and
$(top_distdir).