Fix gyptest-xcode-gcc.py when using Xcode 5.0.2

With Xcode 5.0.2, gcc is now a special version of clang and it no
longer supports the -Wno-invalid-offsetof. Skip the test then.

BUG=381
R=mark@chromium.org

Review URL: https://codereview.chromium.org/83113002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1809 78cadc50-ecff-11dd-a971-7dbc132099af