The current source code upgrade of giflib (4.1.6 -> 4.2.0) introduces a
build dependency on both the xml conversion utility "xmlto" and local
copies of Docbook 4.1.2 DTDs.  A "textproc/xmlto" dependency has been
added to the port Makefile but not one for local copies of the DTDs.
Rather, a work around was introduced in the port that simply disables
xml validation.  I disagree with this solution because it suppresses a
useful feature the developer included that we can easily support.

The current problem arises because "xmlto", a shell script, is hard
coded to invoke "xmllint" with a "--nonet" option.  This prohibits
"xmllint" from using HTTP to fetch required DTDs if local copies are
unavailable.  I don't know the reason for "xmlto" insisting on this
behavior but we can easily accommodate it by adding a build dependency
for "textproc/docbook-sk".  This will install the required DTDs locally
and allow the intended validation.

Fix: The attached patch adds a build dependency to the port Makefile for
"textproc/docbook-sk" and removes the xml validation suppression that is
no longer needed.
How-To-Repeat: 
Try to install graphics/giflib-4.2.0 with the following conditions:

  - Remove the "--skip-validation" line in graphics/giflib/Makefile
    (v1.53).  This was recently introduced by PORTREVISION=2..
  - No "/usr/local/share/xml/docbook/4.1.2" directory.  This is the
    prescribed location for local copies of the required DTDs.