title: Firebase Xcode linker command error using Firebase Unity SDK
body: <p>Firebase Unity SDK 1.1.1.
  Unity 5.5.0p4
  XCode 8.2.1</p>
  
  <p>When using Authentication and Database from Firebase I get the following error when building the project in XCode :</p>
  
  <blockquote>
    <p>Undefined symbols for architecture arm64:<br>
    "_OBJC_CLASS_$_FIRGoogleAuthProvider", referenced from:
          objc-class-ref in libAuth.a(credential_ios_ef8c3cf45c3329a5e5902f99026c639a.o)<br>
    "_OBJC_CLASS_$_FIRGitHubAuthProvider", referenced from:
          objc-class-ref in libAuth.a(credential_ios_ef8c3cf45c3329a5e5902f99026c639a.o)<br>
    "_OBJC_CLASS_$_FIREmailPasswordAuthProvider", referenced from:
          objc-class-ref in libAuth.a(credential_ios_ef8c3cf45c3329a5e5902f99026c639a.o)<br>
    "_OBJC_CLASS_$_FIRFacebookAuthProvider", referenced from:
          objc-class-ref in libAuth.a(credential_ios_ef8c3cf45c3329a5e5902f99026c639a.o)<br>
    "_OBJC_CLASS_$_FIRApp", referenced from:
          objc-class-ref in libApp.a(app_ios_c76c7d869e568a9b561ea55e25a7dcc0.o)<br>
    "_OBJC_CLASS_$_FIRAuth", referenced from:
          objc-class-ref in libAuth.a(auth_ios_3c64a79cf1eb3f06f9309f4d8e91ee94.o)<br>
    "_OBJC_CLASS_$_FIRTwitterAuthProvider", referenced from:
          objc-class-ref in libAuth.a(credential_ios_ef8c3cf45c3329a5e5902f99026c639a.o)<br>
    "_OBJC_CLASS_$_FIROptions", referenced from:
          objc-class-ref in libApp.a(app_ios_c76c7d869e568a9b561ea55e25a7dcc0.o) ld: symbol(s) not
    found for architecture arm64 clang: error: linker command failed with
    exit code 1 (use -v to see invocation)</p>
  </blockquote>
  
  <p>Do I miss something in XCode? Or have something to check in Unity?</p>
  
  <p>Thanks!</p>