The X11 tarballs were missing libSM, and (as Tohru Noryuiki pointed out)
had the wrong permissions on some directories.
I've updated  them in the usual place with the following:

  -rw-rw-r--  1 root  wheel  8948655 Oct 27 22:51 xbase.tar.gz
  -rw-rw-r--  1 root  wheel  2201864 Oct 27 22:36 xcomp.tar.gz
  -rw-rw-r--  1 root  wheel  1653080 Oct 27 22:28 xserver.tar.gz

These _should_ work. :-)

Note that these _don't_ have Sean Davidson's README file, but you
still need to add /usr/X11R6/lib do the ldconfig command line, and
create a symlink for the Xserver, as in that README.

I'd be surprised if the xdm confguration worked; it's a standard X11R6
config, which probably won't work with the R5 xservers wihtout
modification..

For now you might be better off keeping an old, working
usr/X11R6/lib/X11/xdm/, and merging it carefully into the one in the
tarball.   The  ./Xservers file is most important. Since the fonts
are `normal' R6 fonts, the simplest thing is probably to start the
X font server, xfs, and to add an fontserver to the Xservers file.
Something like

	/usr/X11R6/bin/X -fp tcp/localhost:7100

or possibly

	/usr/X11R6/bin/X fp+ tcp/localhost:7100

should work fine. (don't forget to start xfs!)