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

third_party/libxml/BUILD.gn

 # Copyright (c) 2013 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.
 
 # Define an "os_include" variable that points at the OS-specific generated
 # headers.  These were generated by running the configure script offline.
 if (is_linux || is_android || is_nacl) {
   os_include = "linux"
 } else if (is_mac || is_ios) {
   os_include = "mac"
@@ skipping 149 matching lines @@
       "/wd4101",  # Unreferenced local variable.
     ]
   } else if (is_mac || is_ios || is_android) {
     # http://www.xmlsoft.org/threads.html says that this is required when using
     # libxml from several threads, which can possibly happen in chrome. On
     # linux, this is picked up by transitivity from pkg-config output from
     # build/linux/system.gyp.
     defines = [ "_REENTRANT" ]
   }
 
-  if (is_clang) {
-    cflags = [
-      # libxml passes `const unsigned char*` through `const char*`.
-      "-Wno-pointer-sign",
-
-      # pattern.c and uri.c both have an intentional `for (...);` /
-      # `while(...);` loop. I submitted a patch to move the `'` to its own
-      # line, but until that's landed suppress the warning:
-      "-Wno-empty-body",
-
-      # debugXML.c compares array 'arg' to NULL.
-      "-Wno-tautological-pointer-compare",
-
-      # threads.c attempts to forward declare a pthread_equal which doesn't
-      # match the prototype in pthreads.h
-      "-Wno-ignored-attributes",
-    ]
-  }
   config("libxml_warnings") {
     if (is_clang) {
-      cflags += [
+      cflags = [
+        # libxml passes `const unsigned char*` through `const char*`.
+        "-Wno-pointer-sign",
+
+        # pattern.c and uri.c both have an intentional `for (...);` /
+        # `while(...);` loop. I submitted a patch to move the `'` to its own
+        # line, but until that's landed suppress the warning:
+        "-Wno-empty-body",
+
+        # debugXML.c compares array 'arg' to NULL.
+        "-Wno-tautological-pointer-compare",
+
+        # threads.c attempts to forward declare a pthread_equal which doesn't
+        # match the prototype in pthreads.h
+        "-Wno-ignored-attributes",
+
         # libxml casts from int to long to void*.
         "-Wno-int-to-void-pointer-cast",
 
         # libxml passes a volatile LPCRITICAL_SECTION* to a function expecting
         # a void* volatile*.
         "-Wno-incompatible-pointer-types",
+
+        # trio_is_special_quantity and trio_is_negative are only
+        # used with certain preprocessor defines set.
+        "-Wno-unused-function",
       ]
     }
   }
   configs += [ ":libxml_warnings" ]
 
   include_dirs = [ "$os_include" ]
 }