Adjusted link flags for Mac OS X

First, glfw3 builds as libglfw3 by default, so we link against that.
Then, we have to link a few frameworks, to get rid of missing symbols.

glfw3 homebrew PR pending: https://github.com/mxcl/homebrew/pull/21620

FWIW, the glfw -> glfw3 move is probably valid for Linux too, but I can't test that right now, so not included.