Derive version numbers from git

Derive version numbers from git

I often have several versions of notmuch compiled and it would be very
helpful to be able to distinguish between them. Git has a very nice
feature to make intermediate numbering automatic and unambiguous so
let's use it here.

For tagged versions, the version is the name of the tag, for
intermediate versions, the unique ID of the commit is appended to the
tag name.

When notmuch is compiled from a release tarball, there is no git
repository and therefore the tarball contains a special file 'version',
which contains the version of release tarball.

To create a new release one has to run 'make release VERSION=X.Y'.