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

third_party/usrsctp/usrsctp.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': {
     'libsctp_target_type%': 'static_library',
   },
   'target_defaults': {
     'defines': [
       'SCTP_PROCESS_LEVEL_LOCKS',
@@ skipping 102 matching lines @@
         'usrsctplib/user_mbuf.h',
         'usrsctplib/user_queue.h',
         'usrsctplib/user_recv_thread.c',
         'usrsctplib/user_recv_thread.h',
         'usrsctplib/user_route.h',
         'usrsctplib/user_socket.c',
         'usrsctplib/user_socketvar.h',
         'usrsctplib/user_uma.h',
         'usrsctplib/usrsctp.h'
       ],  # sources
+      'variables': {
+        'clang_warning_flags': [
+          # atomic_init in user_atomic.h is a static function in a header.
+          '-Wno-unused-function',
+        ],
+      },
       'conditions': [
         ['OS=="linux" or OS=="android"', {
           'defines': [
             '__Userspace_os_Linux',
           ],
           'cflags!': [ '-Werror', '-Wall' ],
           'cflags': [ '-w' ],
         }],
         ['OS=="mac" or OS=="ios"', {
           'defines': [
@@ skipping 32 matching lines @@
           'msvs_disabled_warnings': [ 4002, 4013, 4133, 4267, 4313, 4700 ],
         }, {  # OS != "win",
           'defines': [
             'NON_WINDOWS_DEFINE',
           ],
         }],
       ],  # conditions
     },  # target usrsctp
   ],  # targets
 }