Yes, true. My idea was to treat 10.9 as "Xcode-only" in our current homebrew-speak (valid for 10.8). This means Homebrew will not assume `/usr/lib` and `/usr/include` to be available for compilation but will pass certain flags to the compiler (and set up a bunch of env vars) so that The tools and header inside of Xcode are used.

And people reported that `MacOS.Xcode.prefix` (tested in `brew irb`) points to a dir that contains the includes on 10.9.

Concluding: Whereas on 10.8 is was sufficient to check for clang and make to decide if the the usual unix tools are in `/usr`, on 10.9 we have to check further.