I see, and (as expected) get the same output when I build like that.

I agree that we should teach _osx_support about this. I guess we should add a function that returns the SDK root, which can then be used in distutils (and setuptools, ...) instead of looking for -isysroot in CFLAGS.  The new function can than look for the default SDK when there is no sysroot flag.

For 3.10 we should look into cleaning up configure.ac, it contains some logic that is no longer valid or necessary (such as looking for the architectures supported by /usr/lib/libSystem.dylib).  We should also avoid adding "-isysroot" to the CFLAGS/LDFLAGS to avoid problems when building on user machines that might have a different version of Xcode.