Idea: a release will be a git tag, distributed as an autogenerated
archive (eg. by GitLab release feature).
This version string must therefore be stored in a revisioned file, so we
use `.version` for this purpose. Before tagging a new release this file
will have to be updated with the new version number.
When building gray the revision (shown in the output headers and usage
screen) will be set to either
- the latest commit hash when building from a git checkout,
- from the .version file when building from release archive.
The implementation is based on whether the .git directory is present.
When building the static library the Nixpkgs stdenv `ar` defaults to not
writing timestamps for the members file. This imply make will always try
to add all members to the archive, which defeats the purpose of the
file-level granularity.
