title: Set macOS target for `swift package generate-xcodeproj`?
body: <p>Is there a way to set the macOS target for <code>swift package generate-xcodeproj</code> generates Xcode projects? </p>
  
  <p>For example, set default target to "x86_64-apple-macosx10.12".</p>
  
  <p><strong>Background</strong></p>
  
  <p>Currently, the Xcode 9.2 Swift toolchain shows the following versions on the command line: </p>
  
  <pre class="lang-sh prettyprint-override"><code>swift --version
  # Apple Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)
  # Target: x86_64-apple-macosx10.9
  swift package --version
  # Apple Swift Package Manager - Swift 4.0.0-dev (swiftpm-13752)
  </code></pre>
  
  <p>The result of <code>swift package generate-xcodeproj</code> is an Xcode project with a macOS target of 10.10. </p>
  
  <p><em>Note: the generated Xcode 10.10 target is one version up from the swift command line compiler target of 10.9. So, apparently, the default macOS targets for <code>swift</code> and <code>swift package generate-xcodeproj</code> are determined independent of each other.</em></p>
  
  <p>So far, any solution with <a href="https://github.com/apple/swift-package-manager/blob/master/Documentation/PackageDescriptionV4.md" rel="noreferrer">Package.swift</a>, <code>--xcconfig-overrides</code>, or something else has been elusive.</p>