If you have this problem when compiling some port that depends of libxul:

checking which gecko to use... libxul
checking manual gecko home set... checking for compiler -fshort-wchar option... yes
checking whether to enable C++ RTTI... no
checking whether we have a gtk 2 gecko build... configure: error: This program needs a gtk 2 gecko build
===> Script "configure" failed unexpectedly.
Please run the gnomelogalyzer, available from
"http://www.freebsd.org/gnome/gnomelogalyzer.sh", which will diagnose the
problem and suggest a solution. If - and only if - the gnomelogalyzer cannot
solve the problem, report the build failure to the FreeBSD GNOME team at
gnome@FreeBSD.org, and attach (a)
"/usr/ports/x11/yelp/work/yelp-2.30.2/config.log", (b) the output of the
failed make command, and (c) the gnomelogalyzer output. Also, it might be a
good idea to provide an overview of all packages installed on your system
(i.e. an `ls /var/db/pkg`). Put your attachment up on any website,
copy-and-paste into http://freebsd-gnome.pastebin.com, or use send-pr(1) with
the attachment. Try to avoid sending any attachments to the mailing list
(gnome@FreeBSD.org), because attachments sent to FreeBSD mailing lists are
usually discarded by the mailing list software.
*** Error code 1

Stop in /usr/ports/x11/yelp.

===>>> make failed for x11/yelp
===>>> Aborting update

===>>> Update for x11/yelp failed
===>>> Aborting update

Terminated

try:

# portmaster -o www/libxul19 www/libxul