I’ve spent a few days on this one: Adding external headers to Xcode 4.6.3 in the project settings pane didn’t seem to work.
It turns out there are two sets of settings, one for the project, and one for the build target. If you find that your headers aren’t being found, then select the build target in the build settings pane, and you’ll then find another set of header search path selection.
Add the external path (in my case, it was /usr/local/include), and viola! Building this target works!