lib error making xfce4-panel-4.0.4

[Phil Lawrence]

I have a VectorLinux 4.0 laptop (Slackware derived).  I'm installing 
xfce 4.0.4 from source.

I made it through dbh, libxfce4util, libxfcegui4, libxfce4mcs, and 
xfce-mcs-manager.

Now, per the install guide, i try xfce4-panel.

I get this error from make:
grep: /usr/lib/libgtk-x11-2.0.la: No such file or directory

However, i *do* have /usr/X11R6/lib/libgtk-x11-2.0.la

Also, my /etc/ld.so.conf lists /usr/X11R6/lib


Really, I'm quite a beginner at this installing from source game...  Any 
hints?

Thanks,
Phil