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
Fix gyptest-sdkroot when oldest SDK is >= 10.8

When using Mac OS X Mavericks with Xcode 5.0.2, the only SDK installed
are 10.8 and 10.9. Fix the test by using the oldest installed and
supported SDK.

BUG=381
R=mark@chromium.org

Review URL: https://codereview.chromium.org/83073002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1810 78cadc50-ecff-11dd-a971-7dbc132099af
Fix gyptest-xcode-env-order for Xcode 5.0.0

The evaluation bug of bare environment variable has been fixed by
Xcode 5.0.0, so do not check for the failure anymore.

BUG=381
R=mark@chromium.org

Review URL: https://codereview.chromium.org/83173004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1811 78cadc50-ecff-11dd-a971-7dbc132099af