Avoid undefined variable in gyp files.

tools/gyp/configurations_xcode.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # The purpose of this file and others in this directory is to simulate
 # the Chromium build enviroment. This file is included in all GYP
 # files that are used by the Dart project.
 
 # READ BEFORE EDITING:
 # Do not add Dart VM specific configuration to this file. Instead,
 # modify runtime/tools/gyp/runtime-configurations.gypi.
 
 {
   'variables': {
     'xcode_gcc_version%': '<!(python <(DEPTH)/tools/gyp/find_mac_gcc_version.py)',
   },
   'target_defaults': {
     'configurations': {
       'Dart_Macos_Base': {
         'abstract': 1,
         'xcode_settings': {
           'GCC_VERSION': '<(xcode_gcc_version)',
-          'MACOSX_DEPLOYMENT_TARGET': '<(mac_deployment_target)',
           'GCC_C_LANGUAGE_STANDARD': 'ansi',
           'GCC_ENABLE_CPP_EXCEPTIONS': 'NO', # -fno-exceptions
           'GCC_ENABLE_CPP_RTTI': 'NO', # -fno-rtti
           'GCC_DEBUGGING_SYMBOLS': 'default', # -g
           'GCC_GENERATE_DEBUGGING_SYMBOLS': 'YES', # Do not strip symbols
           'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES', # -fvisibility=hidden
           'GCC_INLINES_ARE_PRIVATE_EXTERN': 'YES', # -fvisibility-inlines-hidden
           'GCC_WARN_NON_VIRTUAL_DESTRUCTOR': 'YES', # -Wnon-virtual-dtor
           'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES', # -Werror
           'WARNING_CFLAGS': [
@@ skipping 52 matching lines @@
       },
       'Dart_Macos_Debug': {
         'abstract': 1,
       },
       'Dart_Macos_Release': {
         'abstract': 1,
       },
     },
   },
 }