meson: Determine project version from date

Numbers are arbitrary, and we've never successfully made it to version 1.0 after like 12 years or something.

Furthermore, accountsservice has a very slow release schedule at present and some distros hate building from git.

So, I'd like to start generating tarballs more regularly.

Adopting a version number derived from the date will help facilitate that.

This commit introduces a script to figure out the version number based on the date and the number of commits since the last release.