make generator: make gyp recognize OpenBSD

BUG=311
Review URL: https://codereview.chromium.org/12389082/
Patch from Aaron Bieber <deftly@gmail.com>!

git-svn-id: http://gyp.googlecode.com/svn/trunk@1590 78cadc50-ecff-11dd-a971-7dbc132099af