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

build/config/compiler/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.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/ccache.gni")
 
@@ skipping 922 matching lines @@
     cflags += [
       "/W3",  # Warning level 3.
       "/wd4800",  # Disable warning when forcing value to bool.
       "/wd4267",  # TODO(jschuh): size_t to int.
       "/wd4996",  # Deprecated function warning.
     ]
     defines += [
       "_CRT_NONSTDC_NO_WARNINGS",
       "_CRT_NONSTDC_NO_DEPRECATE",
     ]
+  } else if (is_clang && !is_nacl) {
+    # TODO(thakis): Remove !is_nacl once
+    # https://codereview.webrtc.org/1552863002/ made its way into chromium.
+    cflags += [ "-Wall" ]
   }
 
   if (is_linux) {
     # 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. BSD systems do
     # not support this option, since they are usually using gcc 4.2.1, which
     # does not have this flag yet.
     cflags += [ "-Wno-unused-result" ]
   }
 
@@ skipping 350 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }