Fall back to asciidoc6 html backend if html4 doesn't work.

This reenables building documentation on Debian sarge,
which broke after 9346a7fb3fe2.

doc/Makefile

@@ -1,21 +1,21 b''
 SOURCES=$(wildcard *.[0-9].txt)
 MAN=$(SOURCES:%.txt=%)
 HTML=$(SOURCES:%.txt=%.html)
 
 all: man html
 
 man: $(MAN)
 
 html: $(HTML)
 
 %: %.xml
 	xmlto man $*.xml
 
 %.xml: %.txt
 	asciidoc -d manpage -b docbook $*.txt
 
 %.html: %.txt
-	asciidoc -b html4 $*.txt
+	asciidoc -b html4 $*.txt || asciidoc -b html $*.txt
 
 clean:
 	$(RM) $(MAN) $(MAN:%=%.xml) $(MAN:%=%.html)