Change the xcode configuration file to use an overwritable variable

tools/gyp/configurations_xcode.gypi

Index: tools/gyp/configurations_xcode.gypi
diff --git a/tools/gyp/configurations_xcode.gypi b/tools/gyp/configurations_xcode.gypi
index bdd99dfd7af4546b13e240bc8ad374a10dd5a482..ce50d18b39035685ff91affe862ce1d79d6f0bc7 100644
--- a/tools/gyp/configurations_xcode.gypi
+++ b/tools/gyp/configurations_xcode.gypi
@@ -12,13 +12,22 @@
 # 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