Actually, aclocal.m4 claims that either lynx, links, or w3m is needed for the xmlto part of the build and simply removing w3m from Build-Depends caused the package to fail to build. I have now used Build-Depends: lynx since that's already installed for loong64 unstable (and everywhere else). So libxdmcp has now successfully been built for loong64.
See https://bugs.debian.org/825146 which suggests that we might want to add an Architecture: all libxdmcp-doc package to avoid this bootstrapping problem. Thank you, Jeremy Bícha
