Version file is generated at build time and stored as _version.py.
To support running uninstalled tools, a fallback version is included as
well. Example output of `--version` option:

```
$ for t in g-ir-{annotation-tool,scanner,doc-tool}; do $t --version; done
g-ir-annotation-tool 1.57.1
g-ir-scanner 1.57.1
g-ir-doc-tool 1.57.1
```