GitLab

Use `export-subst` to get information about the Git commit used to
produce an archive.

Our exact release commits have a subject of the form `CMake <version>`.
Distinguish development on a release branch in commits after such a
version but before the next release by providing Git information.

We now require at least CMake 3.1 to build, and that has the FindGit
module.

Revert the change from commit 7b354baa (CMakeVersion: Set
CMake_VERSION_RC to 0 even in non-rc versions, 2019-07-25) and instead
define a `0` value in `CMake_VERSION_RC` to mean `-rc0`.  This
distinguishes release branch versions prior to the first release
candidate from the first release candidate itself.  It also makes room
for a dedicated "CMake $major.$minor.0-rc1" release commit for `-rc1` as
we have for later release candidates and final releases.

The logic that uses this value already ignores any "false" value, so `0`
is just as good as not being set at all.  Using `0` for this role makes
the version components look more symmetric and reduces the number of
edits needed when creating releases.