General configuration advice: configure with --pdump, and avoid
Motif.
This did the trick. Everything seemed to build without problems, and
I am using 21.4.9 right now.
*** XEmacs build fails inexplicably.
Inexplicable failures are commonly due to backwards-incompatible
changes in the OS. Try `./configure --pdump ...'.
Seems like a good idea to me. ;)
Thanks for your help,
Michael