This is the classic Unix and OpenVMS Common Desktop Environment (which KDE essentially cloned, before CDE was open-sourced). It is released under the terms described in the COPYING file (LGPL v2.1 license for the code, plus CC-SA for the media). You may reuse and redistribute this code under the terms of this license. See the COPYING file for details.
This release of CDE under a new opensource license is numbered starting at version 2.2.0.
CDE's original release can be found at the sourceforge cdesktopenv project.
And cloned from:
git clone git://git.code.sf.net/p/cdesktopenv/code cdesktopenv-code
The git repository will always be more up to date than the downloadable tarballs we make available, so if you have problems, please try the latest version from git master.
Complete build and installation instructions can be found on the CDE Wiki
Please go there and read the appropriate section(s) for your OS (Linux or FreeBSD/OpenBSD/NetBSD currently). There are a variety of dependencies that must be met, as well as specific set up steps required to build.
Do not expect to just type 'make' and have it actually work without meeting the prerequisites and following the correct steps as spelled out on the wiki. :)
There are also a lot of other documents and information there that you might find useful.
You can join the development mailing list here:
https://lists.sourceforge.net/lists/listinfo/cdesktopenv-devel
There is (was?) also CDE IRC channel, #cde on chat.freenode.net.
Bug reports and patches encouraged (but to the original sourceforge site / irc / mailing list).