Build libvpx with -Wno-unused-function.

libvpx.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.
 {
   'variables': {
     'libvpx_build_vp9%': 1,
     'libvpx_source%': 'source/libvpx',
     # Disable LTO for neon targets
     # crbug.com/408997
     'use_lto%': 0,
@@ skipping 47 matching lines @@
         ['OS=="win" and buildtype=="Official"', {
           # Do not set to 'size', as it results in an error on win64.
           'optimize' :'speed',
         }],
       ],
       'clang_warning_flags': [
         # libvpx heavily relies on implicit enum casting.
         '-Wno-conversion',
         # libvpx does `if ((a == b))` in some places.
         '-Wno-parentheses-equality',
+        # libvpx has many static functions in header, which trigger this warning
+        '-Wno-unused-function',
       ],
       'clang_warning_flags_unset': [
         # libvpx does assert(!"foo"); in some places.
         '-Wstring-conversion',
       ],
     },
   },
   'conditions': [
     ['target_arch=="ia32"', {
       'includes': ['libvpx_srcs_x86_intrinsics.gypi', ],
@@ skipping 202 matching lines @@
                   '-fstack-protector-all',  # Implies -fstack-protector
                 ],
               },
             }],
           ],
         },
       ],
     }],
   ],
 }