Makefile: generate descriptions on-the-fly

Checking in the generated descriptions files makes few things simpler,
but causes pain for pull requests: (1) PRs that touch descriptions
_always_ conflict, (2) PRs are large and harder to review,
(3) people sometimes forget to add auto-generated files.

The proposed way does not require us to hardcode lots of dependencies
in the Makefile (which is nice) and seem to work.
Let's see how it works.

The main contributor-visible consequence is that the auto-generated
files do not need to be checked-in now.

Credit for figuring the Makefile magic goes to @melver.

Fixes #1291

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