title: xcode use DropboxSDK.framework objective-c failed when building for simulator
body: <p>I designed a app for files access include <code>dropbox</code> on <code>iphone</code> in <code>xcode9</code> with <code>DropboxSDK.framework</code>. The code ran on device is fine but will happen following error message when building code for simulator. Is there any solutions to solve following problem?</p>
  
  <p>Undefined symbols for architecture x86_64:</p>
  
  <pre><code>"_OBJC_CLASS_$_DBFILESFolderMetadata", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  "_OBJC_CLASS_$_DBFILESThumbnailSize", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  "_OBJC_CLASS_$_DBFILESFileMetadata", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  "_OBJC_CLASS_$_DBClientsManager", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
    objc-class-ref in AppDelegate.o
  "_OBJC_CLASS_$_DBFILESThumbnailFormat", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  "_OBJC_CLASS_$_DBFILESCommitInfo", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  "_OBJC_CLASS_$_DBFILESDeletedMetadata", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  "_OBJC_CLASS_$_DBFILESUploadSessionCursor", referenced from:
    objc-class-ref in ISDropboxFileSystem.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>