Warning: X11 not installed.
You don't have X11 installed as part of your Xcode installation.
This isn't required for all formulae, but is expected by some.

Your pkg-config is not checking "/usr/X11/lib/pkgconfig" for packages.
Earlier versions of the pkg-config formula did not add this path
to the search path, which means that other formula may not be able
to find certain dependencies.

To resolve this issue, re-brew pkg-config with:
  brew rm pkg-config && brew install pkg-config