Recursive pkgconfig.path on Mac/Homebrew

Before, we would set the `pkgconfig.path` variable on Mac/Homebrew to look for .pc files in the package-specific directories in the Homebrew prefix's opt directory.  This ensures that we can find any keg-only dependencies.  Turns out this is not enough; we need to do that recursively for any dependency reachable from the current package, not just for its direct dependencies.