Fix Git tag -> version string conversion on CMake 2.8.0.

This version is notably shipped with Ubuntu 10.04 LTS, used
for building the Linux release binaries.
Simplify git_describe to work on CMake 2.8.0.

FindGit doesn't seem to work as expected on Ubuntu 10.04
(GIT-NOTFOUND even though Git is installed and available
on the $PATH).

This implementation might be overly simplistic and it
should probably use find_program() to locate Git. We need
to revisit this after the 0.12.0 release is out.