Compile external code with -Wall.

build/external_code.gypi

 # 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.
 
 {
   'conditions': [
     [ 'OS=="linux"', {
       'target_defaults': {
         'cflags!': [
-          '-Wall',
           '-Werror',
         ],
       },
     }],
     [ 'OS=="win"', {
       'target_defaults': {
         'defines': [
           '_CRT_SECURE_NO_DEPRECATE',
           '_CRT_NONSTDC_NO_WARNINGS',
           '_CRT_NONSTDC_NO_DEPRECATE',
@@ skipping 11 matching lines @@
     [ 'OS=="mac"', {
       'target_defaults': {
         'xcode_settings': {
           'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO',
           'WARNING_CFLAGS!': ['-Wall'],
         },
       },
     }],
   ],
 }