title: How do you build iOS specific packages using SPM?
body: <p>I am using Swift Package Manager that doesn't having a xcodeproject file associated with it and I get an error when building through he terminal. When I call the <code>swift build</code> command I get an error that the MacOS build failed. The package I'm building doesn't support MacOS (It uses UIKit), but only iOS. I can't figure out a way to call the command to only specify that the build is targeted for iOS. I've Google searched around with no luck. Does anybody know the correct syntax if it exists to build an SPM package for iOS from the terminal?</p>
  
  <p>The version of Swift I'm using is: "Apple Swift version 5.2.4 (swiftlang-1103.0.32.9 clang-1103.0.32.53)"</p>
  
  <p>I have specified the platform in the Package.swift file</p>
  
  <pre><code>let package = Package(
      name: "Package",
      platforms: [.iOS(.v10), ],
      products: [
  ...
  </code></pre>
  
  <p>I have created a sample project on github <a href="https://github.com/mike011/Swift-Package-Manager-Example" rel="noreferrer">https://github.com/mike011/Swift-Package-Manager-Example</a>. When I run <code>swift build</code> it fails with </p>
  
  <p><code>/git/Swift-Package-Manager-Example/Sources/Swift-Package-Manager-Example/iOSSpecificFile.swift:9:8: error: no such module 'UIKit'
  import UIKit
         ^
  /git/Swift-Package-Manager-Example/Sources/Swift-Package-Manager-Example/iOSSpecificFile.swift:9:8: error: no such module 'UIKit'
  import UIKit
         ^
  </code></p>