macOS: crcmod not installed? (gsutil rsync)

When running a gsutil rsync from a cloud storage bucket to a local directory, I got the following warning:

> WARNING: gsutil rsync uses hashes when modification time is not available at
> both the source and destination. Your crcmod installation isn't using the
> module's C extension, so checksumming will run very slowly. If this is your
> first rsync since updating gsutil, this rsync can take significantly longer than
> usual. For help installing the extension, please see "gsutil help crcmod".

Note that the documentation in gsutil help crcmod indicates that macOS should include this by default, quoting:

> gsutil distributes a pre-compiled version of crcmod for macOS, so you shouldn't
> need to compile and install it yourself. If for some reason the pre-compiled
> version is not being detected, please let the Google Cloud Storage team know

So I'm filing an issue as directed.

macOS 10.15.6
Cloud SDK 303.0.0 (core libraries 2020.07.24)