The current method doesn't work with Xcode 4 and up, since they no
longer have a $DEVELOPER_DIR/SDKs directory. Using xcrun and xcodebuild
works all the way back to Xcode 3 on OS X 10.6 Snow Leopard, if not
earlier.

Change-Id: I7126f2fb4a8f1d6e46f921e70bbd090f00ce3d36