Guard against configurations without 'xcode_settings'

e.g. test/generator-output/src/prog1.gyp

TEST=Run
    ./gyptest.py -f xcode-ninja test/generator-output/gyptest-top-all.py

Expected to fail with
    OSError: [Errno 13] Permission denied: 'prog1.ninja.xcworkspace'
(instead of KeyError: 'xcode_settings')

R=sdefresne@chromium.org, justincohen@chromium.org

Review URL: https://codereview.chromium.org/428913002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1960 78cadc50-ecff-11dd-a971-7dbc132099af
Pull in relevant targets when using xcode-ninja

The tests gyptest-actions, gyptest-copies and gyptest-rules fail when
using the xcode-ninja generator because xcode-ninja (by design) only
creates targets for proper executables. In order for these three tests
to pass, the targets they rely on need to be included explicitly.

BUG=gyp:442

TEST=Run
    ./gyptest.py -f xcode-ninja test/generator-output/

Expected: all tests pass (instead of failing the three mentioned above).

R=sdefresne@chromium.org, justincohen@chromium.org

Review URL: https://codereview.chromium.org/423753003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1961 78cadc50-ecff-11dd-a971-7dbc132099af