The GNU Autotools tests for whether futimens and utimensat are available
reliably come to incorrect conclusions on 10.12 with the 10.13 SDK in
Xcode 9. This overrides its decisions by forcing the right answer
in superenv using ac_cv_func_* environment variables and setting them to
"no" on 10.12.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.