How and why does the compiler replace extern functions with a null function?

Consider the following code in linux/drivers/usb/hid-core.c:

static void hid_process_event (struct hid_device *hid,
                                struct hid_field *field,
                                struct hid_usage *usage,
                                __s32 value)
 {
    hid_dump_input(usage, value);
    if (hid->claimed & HID_CLAIMED_INPUT)
          hidinput_hid_event(hid, field, usage, value);
 #ifdef CONFIG_USB_HIDDEV
    if (hid->claimed & HID_CLAIMED_HIDDEV)
          hiddev_hid_event(hid, usage->hid, value);