Change the xcode configuration file to use an overwritable variable

Change the xcode configuration file to use an overwritable variable
for the xcode gcc version.

This allows me to build with xcode 4 without a local patch to
build files by just creating overwrites in my ~/.gyp/include.gypi:

{
  'variables': {
    'xcode_gcc_version': 'com.apple.compilers.llvmgcc42',
    'mac_sdk': '10.7',
    'mac_deployment_target': '10.7',
  }
}


R=ahe@google.com,iposva@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=14894

[ahe, 2012-11-14 13:43:25]

LGTM!