[Refactor] Extract #custom_build_settings from TargetInstaller#add_target

This allows to extend the custom build settings in the concrete target-class specific subclasses.

lib/cocoapods/installer/target_installer.rb

@@ -51,26 +51,32 @@ module Pod
           configuration = @native_target.add_build_configuration(bc_name, type)
         end
 
+        @native_target.build_configurations.each do |configuration|
+          configuration.build_settings.merge!(custom_build_settings)
+        end
+
+        target.native_target = @native_target
+      end
+
+      # Returns the customized build settings which are overridden in the build
+      # settings of the user target.
+      #
+      # @return [Hash{String => String}]
+      #
+      def custom_build_settings
         settings = {}
+
         if target.archs
           settings['ARCHS'] = target.archs
         end
 
-        if target.is_a? AggregateTarget
-          settings['PODS_ROOT'] = '$(SRCROOT)'
-        end
-
         if target.requires_frameworks?
           settings['PRODUCT_NAME'] = target.product_module_name
         else
           settings.merge!('OTHER_LDFLAGS' => '', 'OTHER_LIBTOOLFLAGS' => '')
         end
 
-        @native_target.build_configurations.each do |configuration|
-          configuration.build_settings.merge!(settings)
-        end
-
-        target.native_target = @native_target
+        settings
       end
 
       # Creates the directory where to store the support files of the target.

lib/cocoapods/installer/target_installer/aggregate_target_installer.rb

@@ -32,6 +32,13 @@ module Pod
 
       private
 
+      def custom_build_settings
+        settings = {
+          'PODS_ROOT'          => '$(SRCROOT)',
+        }
+        super.merge(settings)
+      end
+
       # Creates the group that holds the references to the support files
       # generated by this installer.
       #