I have been preaching this for so long to other teams, I really better
clean up my own backyard: Make sure that you can run

    moc --version

and get a somewhat useful indication what version that is. This is
important for when random people use `moc` from random versions of SDK
or – later – build it themselves and then provide bug reports.

When building locally, with `.git` around, this now injects the git
revision:

    $ ./moc --version
    Motoko compiler (revision 9abae15e-dirty)

(the `-dirty` indicates that my work-tee is not clean).

When `.git` is _not_ available, as typially (and rightfully) when
building with nix, it looks if Nix’s `$out` variable is set, and
includes that:

    ~/dfinity/motoko $ $(nix-build -A moc)/bin/moc --version
    Motoko compiler (revision xx6fmamdh5bjph9law6x02d7f4hw8f84-moc-bin)

This is less useful for end-users, but it still allows us to (hopefully)
identify the version, in the worst case by running this command on all
revisions to find the right one:

    $ nix-store --query --outputs $(nix-instantiate -A moc-bin)
    /nix/store/xx6fmamdh5bjph9law6x02d7f4hw8f84-moc-bin

Or by querying hydra (as @basvandijk suggests in
dfinity/sdk#383 (comment))

Once we do versioned releases, we can of course extend this to inject
the version number there, and use `git describe --tag` to have
`v1.0-200-9abae15e` like derscriptions.

I _hope_ this will not cause extra recompilations with `dune`, and will
work out-of-the box for everyone, but we’ll see.

so that it will hopefully not be picked up as a “dependency”

so that it does not get picked up by `common.lib.standaloneRust`