Somehow a git (tarball?[1]) checkout of Autoconf sometimes manages to get a version of UNKNOWN into its .version file. Presumably that is on systems without git installed (or listed as dependency). See on the hydra build: <http://hydra.nixos.org/build/256086>
My point is, the error that follows is quite unintuitive, and the issue itself is worth fixing, too, I guess. Maybe store in git a file containing at least the latest stable release number, so we can fall back to something like 2.65-dirty rather than UNKNOWN? Thanks, Ralf [1] git/cgit allows you to pull snapshot tarballs containing the files of some git commit.
