Improve version generation at build time

This branch brings two improvements to how tinc determines its own version and build time:

- It makes sure the build date and time are regenerated every time tinc is built, even if there are no changes, so that the date and time returned by `tincd --version` is always accurate.
- It moves from hardcoded version numbers to autogenerated ones using `git describe` to ease maintenance and make `tincd --version` output more useful.