Add last resort version string in scripts/version

[nicowilliams]

Ugh. We do already include some built sources (e.g., outputs of Bison and Flex, the prebuilt man page), so why not a bit more? Well, I'm not going that far -- I'm not including src/version.h here, just hard-coding a last resort version string into scripts/version.

This PR is really about exploring what, if anything we might do about these two issues:

1. GitHub insists on including a tarball of the workspace of the repo checked out at the release tag when making releases. That tarball does not include .git, so scripts/version can't figure out the current version being built.
2. Running autoconf/autoreconf in the dist tarballs we publish in our releases clobbers the src/version.h included in the tarball, and once again there is no .git so scripts/version can be used by Makefile to recover from this.

This PR adds a fallback at the price of having one more thing to update at release time. It might be better instead to commit src/version.h, but that's also going to be a pain since we'd have to do that at every commit!

[wader]

Seems like we're not alone with the confusing "Source code" download links https://github.com/orgs/community/discussions/6003. I subscribed to the thread to keep an eye, i guess we would like to disable them if it was possible?

Could an option to be fallback generate some version string that made it very clear something is not right?

[nicowilliams]

Could an option to be fallback generate some version string that made it very clear something is not right?

Oh, that's an interesting idea. I like it.

[nicowilliams]

Another option is to have .github/workflows/ci.yml make a commit to set the version (thus no need to worry about forgetting to do it manually).