Darwin: Use xcrun to get the path to the SDK

Recent versions of Xcode don't install headers in /usr/include but
in a directory like

  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include

So use that instead, falling back to /usr/include in case of an older
version of Xcode.