By using the native access functions you assert that you know what you're doing and how to fix problems caused by using them. If you don't, you shouldn't be using them.
Before the inclusion of glfw3native.h, you must define exactly one window system API macro and exactly one context creation API macro. Failure to do this will cause a compile-time error.
The available window API macros are:
GLFW_EXPOSE_NATIVE_WIN32
GLFW_EXPOSE_NATIVE_COCOA
GLFW_EXPOSE_NATIVE_X11
The available context API macros are:
GLFW_EXPOSE_NATIVE_WGL
GLFW_EXPOSE_NATIVE_NSGL
GLFW_EXPOSE_NATIVE_GLX
GLFW_EXPOSE_NATIVE_EGL
These macros select which of the native access functions that are declared and which platform-specific headers to include. It is then up your (by definition platform-specific) code to handle which of these should be defined.