Change the xcode configuration file to use an overwritable variable

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. This includes V8's GYP
 # files.
 
 # READ BEFORE EDITING:
 # Do not add Dart VM specific configuration to this file. Instead,
 # modify runtime/tools/gyp/runtime-configurations.gypi.
 
 {
+  'variables': {
+    # To switch to the LLVM based backend create a ~/.gyp/include.gypi
+    # including:
+    #
+    # {
+    #   'variables': {
+    #     'xcode_gcc_version': 'com.apple.compilers.llvmgcc42',
+    #   }
+    # }
+    'xcode_gcc_version%': '4.2',
+  },
   'target_defaults': {
     'configurations': {
       'Dart_Base': {
         'xcode_settings': {
-          # To switch to the LLVM based backend change the two lines below.
-          #'GCC_VERSION': 'com.apple.compilers.llvmgcc42',
-          'GCC_VERSION': '4.2',
+          'GCC_VERSION': '<(xcode_gcc_version)',
           '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 16 matching lines @@
           'GCC_CW_ASM_SYNTAX': 'NO', # Remove -fasm-blocks.
 
           'GCC_ENABLE_PASCAL_STRINGS': 'NO',
           'GCC_ENABLE_TRIGRAPHS': 'NO',
           'PREBINDING': 'NO',
         },
       },
     },
   },
 }