Fix Xcode 8 builds (again) (#3695)

The fix in #3647 was essentially undone in #3648 because even though the
podspec no longer depends on FirebaseCoreDiagnostics, adding in local
pod declarations for these brought them back into the project.

This change removes local pod declarations for the dependencies that
break under Xcode 8 as well.

Author: wilhuff

Firestore/Example/Podfile

@@ -35,6 +35,15 @@ def use_local_sources()
   return ENV.has_key?('USE_LOCAL_SOURCES') || ENV['TRAVIS_PULL_REQUEST'] == 'false'
 end
 
+def xcode_major_version()
+  version = `xcodebuild -version`
+  if version =~ /^Xcode\s*(\d+)\./
+    return $1.to_i
+  else
+    raise "Could not find Xcode version in '$version'"
+  end
+end
+
 # Adds a `pod name, :path => ../..` declaration to use local sources if the
 # Podfile has been configured to operate that way.
 def maybe_local_pod(name)
@@ -57,11 +66,28 @@ def configure_local_pods()
   # Pull in local sources conditionally.
   maybe_local_pod 'FirebaseAuth'
   maybe_local_pod 'FirebaseAuthInterop'
-  maybe_local_pod 'FirebaseCoreDiagnostics'
-  maybe_local_pod 'FirebaseCoreDiagnosticsInterop'
-  maybe_local_pod 'GoogleDataTransport'
-  maybe_local_pod 'GoogleDataTransportCCTSupport'
   maybe_local_pod 'GoogleUtilities'
+
+  if xcode_major_version() >= 9
+    # Firestore still compiles with Xcode 8 to help verify general conformance
+    # with C++11 by using an older compiler that doesn't have as many
+    # extensions from later versions of the language. However, Firebase as a
+    # whole does not support this environment and @available checks in
+    # GoogleDataTransport would otherwise break this build.
+    #
+    # Firestore doesn't depend on GoogleDataTransport directly--it comes in as
+    # a dependency of FirebaseCoreDiagnostics. Luckily, FirebaseCore does not
+    # strongly depend on this, so we can edit the dependency out in the podspec
+    # and then avoid adding those dependencies here to prevent CocoaPods from
+    # importing it.
+    #
+    # This list should include the transitive closure of all dependencies of
+    # FirebaseCoreDiagnostics, except GoogleUtilities which we otherwise need.
+    maybe_local_pod 'FirebaseCoreDiagnostics'
+    maybe_local_pod 'FirebaseCoreDiagnosticsInterop'
+    maybe_local_pod 'GoogleDataTransport'
+    maybe_local_pod 'GoogleDataTransportCCTSupport'
+  end
 end
 
 target 'Firestore_Example_iOS' do