Fall back to asciidoc6 html backend if html4 doesn't work.
This reenables building documentation on Debian sarge,
which broke after 9346a7fb3fe2.
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions