The build of CDE on Slackware has only been tested on Slackware 15.0. It may or may not work on other versions. Here are the modifications needed to get CDE built and started.
A Slackbuild for Slackware 15 can be found here: https://slackbuilds.org/repository/15.0/desktop/cde/
I found I needed the following packages to build
But even that is not a sufficient set to run the X server itself