gtk-doc needs docbook-xsl to build

Error before installing docbook-xsl manually:

==> Downloading http://ftp.gnome.org/pub/GNOME/sources/gtk-doc/1.18/gtk-doc-1.18
Already downloaded: /Library/Caches/Homebrew/gtk-doc-1.18.tar.bz2
==> ./configure --prefix=/usr/local/Cellar/gtk-doc/1.18 --disable-scrollkeeper -
checking for XML catalog (/usr/local/etc/xml/catalog)... found
checking for xmlcatalog... /usr/local/opt/libxml2/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