We want to use flua from the source tree (not the host) during package
build, firstly to protect us from breaking changes in lua or libucl,
and secondly to allow (in future) cross-building of packages from
Linux or macOS.
Since we don't know if the user will be building packages during the
bootstrap phase, and because flua is fairly small and generally useful,
build it as a bootstrap tool unconditionally.
MFC after: 2 weeks
Sponsored by: https://www.patreon.com/bsdivy