Enabled internal versions GLEW and GLFW on OS X.
Disabled -fpermissive compiler flag on OS X.

The flag is not supported on OS X and caused warnings.
Enabled debug output callback on OS X.

It is used by the samples and does not use anything not available on OS
X, so there is no need to disable it during compilation.
Removed superfluous link_directories call.

The add_dependencies and target_link_libraries calls set up all
necessary links between the projects.