Using any SBo through PPM is experimental and there is no guarantee that it will run. In this example you chose from the 12.2 repo. At that point in time (12.2 slackware) kde-3.5.12 was being used and kde-config existed in this environment. It doesn't exist under KDE4
When using KDE4 you would be best to pull apps only from slackware-13.37 to ensure they are compatible with KDE4.
You could try: ln -s /usr/bin/kde4-config /usr/bin/kde-config
This will certainly get you passed the problem you are having during compile ... but i would bet you will hit another wall further in the compile.
How do i become super user?
Wear your underpants on the outside and put on a cape.