These functions are non portable. They may expose some part of the USB API on one OS or perhaps a couple, but not all. They are all marked with the string _np at the end of the function name.
A C preprocessor macro will be defined if the function is implemented. The form is LIBUSB_HAS_ prepended to the function name, without the leading "usb_", in all caps. For example, if usb_get_driver_np is implemented, LIBUSB_HAS_GET_DRIVER_NP will be defined.