[installer] link using LDFLAGS instead of product reference

Since we want to do conditional linking, the list of Pods to link with 
needs to be provided by an xcconfig file. Only way to do that is to
remove the linking responsibility from Xcode and give it to ld directly.

lib/cocoapods/generator/xcconfig/aggregate_xcconfig.rb

@@ -67,6 +67,11 @@ module Pod
               XCConfigHelper.add_library_build_settings(vendored_library, @xcconfig, target.sandbox.root)
             end
           end
+
+          # This is how the Pods project now links with dependencies, instead of a "Link with Libraries" build phase
+          @xcconfig.merge!({
+            'OTHER_LDFLAGS' => "-l#{pod_target.name}"
+          })
         end
 
         # TODO Need to decide how we are going to ensure settings like these

lib/cocoapods/installer.rb

@@ -114,7 +114,7 @@ module Pod
         install_file_references
         install_libraries
         set_target_dependencies
-        link_aggregate_target
+        # note: linking with pod libraries is now done by aggregate_xcconfig's OTHER_LDFLAGS
         run_post_install_hooks
         write_pod_project
         write_lockfiles
@@ -379,21 +379,6 @@ module Pod
       end
     end
 
-    # Links the aggregate targets with all the dependent libraries.
-    #
-    # @note   This is run in the integration step to ensure that targets
-    #         have been created for all per spec libraries.
-    #
-    def link_aggregate_target
-      aggregate_targets.each do |aggregate_target|
-        native_target = aggregate_target.target
-        aggregate_target.pod_targets.each do |pod_target|
-          product = pod_target.target.product_reference
-          native_target.frameworks_build_phase.add_file_reference(product)
-        end
-      end
-    end
-
     # Writes the Pods project to the disk.
     #
     # @return [void]