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

third_party/libxml/libxml.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': {
     'conditions': [
       # Define an "os_include" variable that points at the OS-specific generated
       # headers.  These were generated by running the configure script offline.
       ['os_posix == 1 and OS != "mac" and OS != "ios"', {
@@ skipping 199 matching lines @@
               '-Wno-empty-body',
               # debugXML.c compares array 'arg' to NULL.
               '-Wno-tautological-pointer-compare',
               # See http://crbug.com/138571#c8
               '-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',
             ],
           },
           'include_dirs': [
             '<(os_include)',
             '<(os_include)/include',
             'src/include',
           ],
           'dependencies': [
             '../icu/icu.gyp:icuuc',
             '../zlib/zlib.gyp:zlib',
@@ skipping 37 matching lines @@
             }],
           ],
         }],
         ['OS == "ios"', {
           'toolsets': ['host', 'target'],
         }],
       ],
     },
   ],
 }