Here's a patch that updates the CMake build scripts to produce a source tarball. After running cmake in your build directory, "make package_source" will produce the source tarball.
The only caveat is that CPack doesn't try to be smart about what to include in the tarball; basically anything in the top-level source directory is included. I've added some ignore patterns to keep out some of the obvious riff-raff like backup files, and the CMake build directory. But to produce a truly pristine source tarball, you should make sure to keep the source directory itself pristine.