Implement something similar to version_source_revision for the Galera library

It would be useful to know which exact version of the Galera library is being used, especially when testing unreleased development versions of the library. This could save a lot of time when diagnosing failures that occur in a continuous integration system.

The MariaDB server exposes version_source_revision. Could the Galera library source revision be exposed in some way?

(Note: git submodule revision numbers are implied by version_source_revision. I am referring to libraries that are built or loaded independently of the source code revision of the server.)