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

third_party/usrsctp/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/crypto.gni")
 
 config("usrsctp_config") {
   include_dirs = [
     "usrsctplib",
     "usrsctplib/netinet",
@@ skipping 66 matching lines @@
   ]
 
   defines = [
     "SCTP_PROCESS_LEVEL_LOCKS",
     "SCTP_SIMPLE_ALLOCATOR",
     "__Userspace__",
 
     # "SCTP_DEBUG", # Uncomment for SCTP debugging.
   ]
 
+  config("usrsctp_warnings") {
+    if (is_clang) {
+      # atomic_init in user_atomic.h is a static function in a header.
+      cflags = [ "-Wno-unused-function" ]
+    }
+  }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   configs += [ "//build/config/compiler:no_incompatible_pointer_warnings" ]
+  configs += [ ":usrsctp_warnings" ]
 
   public_configs = [ ":usrsctp_config" ]
 
   if (use_openssl) {
     defines += [ "SCTP_USE_OPENSSL_SHA1" ]
   } else {
     defines += [ "SCTP_USE_NSS_SHA1" ]
   }
 
   if (is_linux || is_android) {
@@ skipping 19 matching lines @@
     ]
     configs -= [ "//build/config/win:winver" ]
   } else {
     defines += [ "NON_WINDOWS_DEFINE" ]
   }
 
   deps = [
     "//crypto:platform",
   ]
 }