* build: require unibilium>=2.0
This also ports FindUnibilium to LibFindMacros, which was planned
anyway, and makes the version check easier.
With an older Unibilium our fallback code in `terminfo_from_builtin`
will not work (because it assumes the new data structures from 2.0.0 [1]),
and nvim would crash later because of `ut` being NUL.
1: 42f3cdd284