The answer (http://www.freebsd.org/gnome/docs/faq2.html#q2) is toward the end of #3 in the freebsd gnome port team's faq

Essentially there are some libtool archive files hanging about that have outdated information in them. You have to find the archive file and force a portupgrade of the effected packages. So....

# find / -type f -name "*.la" | xargs grep -l /usr/X11R6/lib/libgtk-x11-2.0.la

For each match you find figure out which port it came from using pkg_info. For example, if you found a patch in /usr/X11R6/lib/xyz.la

# pkg_info -W /usr/X11R6/lib/xyz.la

If that command tells you that one or more ports are associated with the package then do:

# portupgrade -f -P portname

for each port.

If it doesn't find a port it is probably safe to delete the .la file in question (to be safe rename it or save it elsewhere).

Get those cleaned up and then you can complete the install.

#portupgrade xfce
(of course my ports' tree is up to date )

I get this error while compiling xfce:

libtool: link: cannot find the library `/usr/X11R6/lib/libgtk-x11-2.0.la'
gmake[2]: *** [xfce-setting-show] Error 1
gmake[2]: Leaving directory `/usr/ports/sysutils/xfce4-mcs-manager/work/xfce-mcs-manager-4.4.0/xfce-setting-show'
gmake[1]: *** [all-recursive] Error 1
gmake[1]: Leaving directory `/usr/ports/sysutils/xfce4-mcs-manager/work/xfce-mcs-manager-4.4.0'
gmake: *** [all] Error 2
*** Error code 2

the library 'libgtk-x11-2.0.la' is in /usr/local/lib instead of /usr/X11R6/lib
I already tried to edit PKG_CONFIG_PATH, in vain.

What can I do ?
thx