gtk-doc couldn't find the docbook-xml DTD in the catalog.

Following the example of textproc/scrollkeeper/Makefile fixed it.

Does devel/buildtool-doc need a similar change?

candor# cvs diff Makefile
Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/textproc/gtk-doc/Makefile,v
retrieving revision 1.14
diff -r1.14 Makefile
20c20
< CONFIGURE_ARGS+=      --with-xml-catalog=${XML_DEFAULT_CATALOG}
---
> CONFIGURE_ARGS+=      --with-xml-catalog=${BUILDLINK_PREFIX.xmlcatmgr}/share/xml/catalog