CMake: use the output of git-describe as version

When you need to test out multiple versions, it's not always clear
which one is installed. When given the DESCRIBE_VERSION option, our
build system will now use git-describe to determine the version,
instead of using the one in version.h.

The major, minor and revision defines stay as they are, but
LIBGIT2_VERSION will be set to this describe string.

---

BOOM! /cc @nulltoken @ben

We could also add a specific vendor string if we wanted, but this should be enough for what we need.