We use `git describe --tags` to construct a version number based on
the current version tag.
The properties VERSION (when using make) and Version (when using
MSBuild) can be explicitly set to use a fixed value instead.
Fixes#13
Now that we no longer recursively call yarn via yarn run and scripts,
we can use a `--mutex network` directive in a .yarnrc file at the root
of the repository to prevent multiple copies of yarn running
concurrently.
