Properly escape Xcode defines containing ' 

BUG=gyp:257
TEST=python gyptest.py -a -f xcode,make

Patch by Jan Svarovsky <honza@google.com>
Review URL: https://codereview.chromium.org/18621003/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1663 78cadc50-ecff-11dd-a971-7dbc132099af