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

third_party/libusb/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.
 
 assert(!is_android && !is_ios)
 
 import("//build/config/features.gni")
 
 config("libusb_config") {
   include_dirs = [ "src/libusb" ]
@@ skipping 28 matching lines @@
     "src/libusb/sync.c",
     "src/libusb/version.h",
     "src/libusb/version_nano.h",
     "src/msvc/config.h",
     "src/msvc/inttypes.h",
     "src/msvc/stdint.h",
   ]
   deps = []
   include_dirs = [ "src/libusb/os" ]
 
+  config("libusb_warnings") {
+    if (is_clang) {
+      # guid_eq in windows_usb.c is unused.
+      cflags = [ "-Wno-unused-function" ]
+      if (is_linux && !use_udev) {
+        cflags += [ "-Wno-pointer-sign" ]
+      }
+    }
+  }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [ ":libusb_warnings" ]
 
   public_configs = [ ":libusb_config" ]
 
   if (is_posix) {
     defines = [
       "DEFAULT_VISIBILITY=",
       "HAVE_GETTIMEOFDAY=1",
       "HAVE_POLL_H=1",
       "HAVE_SYS_TIME_H=1",
       "LIBUSB_DESCRIBE=\"1.0.16\"",
@@ skipping 27 matching lines @@
     defines += [
       "HAVE_LIBUDEV=1",
       "USE_UDEV=1",
     ]
     deps += [ "//build/config/linux:udev" ]
   }
 
   if (is_linux && !use_udev) {
     sources += [ "src/libusb/os/linux_netlink.c" ]
     defines += [ "HAVE_LINUX_NETLINK_H" ]
-    if (is_clang) {
-      cflags += [ "-Wno-pointer-sign" ]
-    }
   }
 
   if (is_win) {
     include_dirs += [ "src/msvc" ]
     sources -= [
       "src/libusb/os/poll_posix.c",
       "src/libusb/os/threads_posix.c",
     ]
     libs = [ "setupapi.lib" ]
   } else {
     include_dirs += [ "src" ]
     sources -= [
       "src/libusb/os/poll_windows.c",
       "src/libusb/os/poll_windows.h",
       "src/libusb/os/threads_windows.c",
       "src/libusb/os/threads_windows.h",
       "src/libusb/os/windows_common.h",
       "src/libusb/os/windows_usb.c",
       "src/libusb/os/windows_usb.h",
       "src/msvc/config.h",
       "src/msvc/inttypes.h",
       "src/msvc/stdint.h",
     ]
   }
 }