Fix gyptest-type-envvars.py for Xcode 5.0.0

With Xcode 5.0.0, the SDKROOT environment variable is always set
when building, even if not set in the Xcode project.

BUG=381
R=mark@chromium.org

Review URL: https://codereview.chromium.org/82763006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1863 78cadc50-ecff-11dd-a971-7dbc132099af