Homebrewable

`bootstrap` no longer depends on `xctool` so that the homebrew formula (#19) doesn’t have to depend on `xctool`.

- [x] sort out the CI failure (thanks whoever did this)
- ~~add brew’s frameworks path to the runpath search paths~~ Doing this in the formula instead, since brew may be installed in nonstandard locations.
Can you please fix the indentation here?