Can't find GUI library

I compiled the GUI library before compiling SDL_net and it says

configure: warning: *** Can’t find the GUI library - not building test
programs
The GUI library can be found at:
http://www.libsdl.org/projects/GUIlib/

How do I tell SDL_net where the GUI Library is?

Slackware 8.1
SDL 1.2.4

Thank you
AresCFH