At this time, the newest SDK version that Audacity can build with
is 10.6.  To build Audacity using Xcode 5.1 or newer, you need
to extract the 10.6 SDK from an earlier version of Xcode.

In the instructions below, Xcode 4.3.3 (for the 10.6 SDK) and
Xcode 5.1.1 will be used.  Xcode 6.1.1 and 6.3 have been verified
to work as well.

1)  Download Xcode 5.1.1 or greater and install it to /Applications
2)  Download Xcode 4.3.3 and open it in Finder
3)  Right click the "Xcode.app" bundle and select "Show Package Contents"
4)  Navigate down the directories to get to:

    Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/

5)  Copy the MacOSX10.6.sdk folder to:

    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/

That's it.  You can now build using the Xcode project in this directory.