title: Where is Xcode's "TOOLCHAINS" set?
body: <p>I upgraded my project from Swift 2.2 to 2.3 (Xcode 7 to 8), and committed everything on one Mac.  When I pulled from git on my other Mac, and built, I get:</p>
  
  <pre><code>Warning: target specifies SWIFT_VERSION = '2.3', but it is overridden by TOOLCHAINS = 'com.apple.dt.toolchain.XcodeDefault'
  </code></pre>
  
  <p>I can't find anything called "TOOLCHAINS" in any file.  What's this warning mean?</p>