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

Review URL: https://codereview.chromium.org//11410084

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@14894 260f80e4-7a28-3924-810f-c04153c831b5

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