Installing gtk-doc from macports failed on my system due to problems with these configure errors:

king for xsltproc... /opt/local/bin/xsltproc
checking for dblatex... no
checking for fop... no
configure: WARNING: neither dblatex nor fop found, so no pdf output from xml
checking for XML catalog (/opt/local/etc/xml/catalog)... found
checking for xmlcatalog... /opt/local/bin/xmlcatalog
checking for DocBook XML DTD V4.3 in XML catalog... not found

This seems to be a problem with the installation of the docbook packages. It was necessary to add the following nextCatalog tags into the catalog:

Afterwards, a new error occured:

king for xsltproc... /opt/local/bin/xsltproc
checking for dblatex... no
checking for fop... no
configure: WARNING: neither dblatex nor fop found, so no pdf output from xml
checking for XML catalog (/opt/local/etc/xml/catalog)... found
checking for xmlcatalog... /opt/local/bin/xmlcatalog
checking for DocBook XML DTD V4.3 in XML catalog... found
checking for DocBook XSL Stylesheets in XML catalog... not found
configure: error: could not find DocBook XSL Stylesheets in XML catalog

This could be fixed by adding another nextCatalog tag: