Download link for the latest version: http://opensource.apple.com/tarballs/libedit/libedit-31.tar.gz
The archive only includes an Xcode based build system, the makefile based system was removed some time ago.
It might be better to use <http://www.thrysoee.dk/editline/libedit-20130611-3.1.tar.gz> instead (which is a distribution of netbsd libedit with a configure script)
I've attached a patch (relative to the tip of the 3.3 branch for now) that teaches build-installer to use a static build of the trysoee distribution of libedit.
The patch is completely untested at the moment, I will be able to build a 10.6/intel installer tonight and testing with the PPC build will have to wait until I get back from EP.
A problem with using libedit unconditionally: we will likely get complaints from users when changing this in a bugfix release, it would be better to only apply this patch to the default branch.