Explicitly pass -Wno-unused-function to the targets that need it.

third_party/libusb/libusb.gyp

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'targets': [
     {
       'target_name': 'libusb',
       'type': 'static_library',
       'sources': [
         'src/config.h',
         'src/libusb/core.c',
         'src/libusb/descriptor.c',
         'src/libusb/hotplug.c',
         'src/libusb/hotplug.h',
         'src/libusb/interrupt.c',
         'src/libusb/interrupt.h',
         'src/libusb/io.c',
         'src/libusb/libusb.h',
         'src/libusb/libusbi.h',
         'src/libusb/strerror.c',
         'src/libusb/sync.c',
         'src/libusb/version.h',
         'src/libusb/version_nano.h',
       ],
+      'variables': {
+        'clang_warning_flags': [
+          # guid_eq in windows_usb.c is unused.
+          '-Wno-unused-function',
+        ],
+      },
       'include_dirs': [
         'src',
         'src/libusb',
         'src/libusb/os',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           'src/libusb',
         ],
       },
@@ skipping 82 matching lines @@
           ],
           'include_dirs': [
             'src/msvc',
           ],
           'msvs_disabled_warnings': [ 4267 ],
         }],
       ],
     },
   ],
 }