Linux/CrOS/Cast/Android (and Mac with gn): Build 3rd-party code with -Wall.

build/common.gypi

 # 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 3122 matching lines @@
       ['chromium_code==0', {
         'variables': {
           'clang_warning_flags': [
             # Lots of third-party libraries have unused variables. Instead of
             # suppressing them individually, we just blanket suppress them here.
             '-Wno-unused-variable',
           ],
         },
         'conditions': [
           [ 'os_posix==1 and OS!="mac" and OS!="ios"', {
-            # We don't want to get warnings from third-party code,
-            # so remove any existing warning-enabling flags like -Wall.
-            'cflags!': [
-              '-Wall',
-              '-Wextra',
-            ],
+            # Remove -Wextra for third-party code.
+            'cflags!': [ '-Wextra' ],
             'cflags_cc': [
               # Don't warn about hash_map in third-party code.
               '-Wno-deprecated',
             ],
             'cflags': [
               # Don't warn about printf format problems.
               # This is off by default in gcc but on in Ubuntu's gcc(!).
               '-Wno-format',
             ],
           }],
+          [ 'os_posix==1 and clang!=1 and OS!="mac" and OS!="ios"', {
+            # When we don't control the compiler, don't use -Wall for
+            # third-party code either.
+            'cflags!': [ '-Wall' ],
+          }],
           # TODO: Fix all warnings on chromeos too.
           [ 'os_posix==1 and OS!="mac" and OS!="ios" and (clang!=1 or chromeos==1)', {
             'cflags!': [
               '-Werror',
             ],
           }],
           [ 'os_posix==1 and os_bsd!=1 and OS!="mac" and OS!="android"', {
             'cflags': [
               # Don't warn about ignoring the return value from e.g. close().
               # This is off by default in some gccs but on by default in others.
@@ skipping 3253 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }