When an SGML doc "foo" is built as HTML, touch "foo.html", so that the
build system can tell it has been built.
# $(JADE) -t sgml -V html-index -d $(SGMLSTYLESHEET) -c $(DOCBOOK_CATALOG) $<
# $(PERL) $(COLLATEINDEX) -N -o index.sgml
$(SGML2HTML) $(SGML2HTML_OPTS) $<
+# touch the .html file so that it is seen to be built
+ @touch $@
%.html : %.tex
@$(RM) $@