fix compile issue on OS X

See https://github.com/codeboost/opencv-node/pull/10/files
fix "lazy symbol binding failed" on OS X

See
https://github.com/peterbraden/node-opencv/commit/f66364f1bbf8b2763b9af3
204bd0cc7a4729dcde
Merge pull request #33 from fgnass/master

fix compile issue on OS X.