Add base_configuration_reference_ fields to XCBuildConfiguration

lib/xcodeproj/project/object/build_configuration.rb

@@ -1,3 +1,5 @@
+require 'xcodeproj/project/object/file_system_synchronized_root_group'
+
 module Xcodeproj
   class Project
     module Object
@@ -25,6 +27,20 @@ class XCBuildConfiguration < AbstractObject
         #
         has_one :base_configuration_reference, PBXFileReference
 
+        # @return [PBXFileSystemSynchronizedRootGroup] an optional reference to a group 
+        #         synchronized with the file system that contains a configuration file (`.xcconfig`).
+        # 
+        # @note   the configuration file relative path must be provided in `base_configuration_reference_relative_path`
+        #
+        has_one :base_configuration_reference_anchor, PBXFileSystemSynchronizedRootGroup
+
+        # @return [String] the relative path of a configuration file (`.xcconfig`)
+        #         inside a group synchronized with the file system.
+        # 
+        # @note   the configuration file group must be provided in `base_configuration_reference_anchor`
+        #
+        attribute :base_configuration_reference_relative_path, String
+
         public
 
         # @!group AbstractObject Hooks