http://code.google.com/p/gyp/issues/detail?id=16
http://code.google.com/p/gyp/issues/detail?id=16

add a generator flag for controlling the parallel building of targets.
Review URL: http://codereview.chromium.org/150154

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp