[Target] Configure as extension only for app extensions

Set the APPLICATION_EXTENSION_API_ONLY build setting if integrating with
an app extension target

lib/cocoapods/installer.rb

@@ -467,6 +467,9 @@ module Pod
     def set_target_dependencies
       frameworks_group = pods_project.frameworks_group
       aggregate_targets.each do |aggregate_target|
+        app_extension = aggregate_target.user_targets.map { |t| t.symbol_type }.include?(:app_extension)
+        set_app_extension_api_only_for_target(aggregate_target) if app_extension
+
         aggregate_target.pod_targets.each do |pod_target|
           unless pod_target.should_build?
             pod_target.resource_bundle_targets.each do |resource_bundle_target|
@@ -477,6 +480,8 @@ module Pod
           end
 
           aggregate_target.native_target.add_dependency(pod_target.native_target)
+          set_app_extension_api_only_for_target(pod_target) if app_extension
+
           pod_target.dependencies.each do |dep|
             unless dep == pod_target.pod_name
               pod_dependency_target = aggregate_target.pod_targets.find { |target| target.pod_name == dep }
@@ -487,6 +492,7 @@ module Pod
 
               next unless pod_dependency_target.should_build?
               pod_target.native_target.add_dependency(pod_dependency_target.native_target)
+              set_app_extension_api_only_for_target(pod_dependency_target) if app_extension
 
               if pod_target.requires_frameworks?
                 product_ref = frameworks_group.files.find { |f| f.path == pod_dependency_target.product_name } ||
@@ -694,6 +700,15 @@ module Pod
       analysis_result.sandbox_state
     end
 
+    # Sets the APPLICATION_EXTENSION_API_ONLY build setting to YES for all
+    # configurations of the given target
+    #
+    def set_app_extension_api_only_for_target(target)
+      target.native_target.build_configurations.each do |config|
+        config.build_settings['APPLICATION_EXTENSION_API_ONLY'] = 'YES'
+      end
+    end
+
     #-------------------------------------------------------------------------#
   end
 end