Fix several problems with gi path warnings
Fix several problems with gi path warnings

* Do not warn when the prefix appears to be a homebrew cellar.
* Fix configure errors when python is not detected
* Fix configure errors when pygobject is installed in prefix but
  not present in the python `sys.path`.
* Use the actual overrides path, instead of the gi module path.

jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips