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

third_party/libxslt/libxslt.gyp

 # Copyright (c) 2009 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': [
       ['os_posix == 1 and OS != "mac"', {
         'os_include': 'linux'
       }],
@@ skipping 96 matching lines @@
             ],
           },
           'msvs_disabled_warnings': [
             # size_t to int conversion.
             4267,
           ],
           'variables': {
             'clang_warning_flags': [
               # libxslt stores a char[3] in a `const unsigned char*`.
               '-Wno-pointer-sign',
+              # xsltDefaultRegion and xsltCalibrateTimestamps are only
+              # used with certain preprocessor defines set.
+              '-Wno-unused-function',
             ],
           },
           'conditions': [
             ['OS!="win"', {'product_name': 'xslt'}],
           ],
         }],
       ],
     },
   ],
 }