Fixes for OS X

I tried using cl-glfw3 on OS X and found several issues.

They are all fixed with this pull request. I can now run the examples using
sbcl-1.1.11 and glfw 3.0.2. I don't have a Linux machine to test this, but the
changes should work on Linux, too.

I'm open to change things if you find something you don't like.