ninja/mac: Fix _XcodeVersion parsing for Xcode 3.2. 

BUG=chromium:295322
Review URL: https://codereview.chromium.org/24243013/

Patch from Trent Apted <tapted@chromium.org>!



git-svn-id: http://gyp.googlecode.com/svn/trunk@1738 78cadc50-ecff-11dd-a971-7dbc132099af