XOpenDisplay Problems?

Hi when i try to run the ./configure script with FreeBSD with the --with-tcl
support i get and error that tells me that X11 Library -lX11 could not be
found so tk support will be disabled. I have freebsd 3.1 could anyone please
tell me what i could do to fix this problem or even where i can find this
XOpenDisplay library i need on my system?

thanks
Brent