osx support

This is based on the original pull request from https://github.com/agl/go-gtk/pull/1 nabla-c0d3 . 

Tested with Homebrew on OSX 10.7:
github.com/cyphunk/go-gtk/example/demo$ PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/local//Cellar/cairo/1.12.14/lib/pkgconfig/ make