mk: Update how the build deals with version labels. #16677
Adds a new configure flag, --release-channel, which determines how the version
number should be augmented with a release label, as well as how the distribution
artifacts will be named. This is entirely for use by the build automation.
--release-channel can be either 'source', 'nightly', 'beta', or 'stable'.
Here's a summary of the affect of these values on version number and
artifact naming, respectively: