[iOS/GN] Allow compilation with system clang.

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/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 989 matching lines @@
 
       # TODO(thakis): This used to be implied by -Wno-unused-function,
       # which we no longer use. Check if it makes sense to remove
       # this as well. http://crbug.com/316352
       "-Wno-unneeded-internal-declaration",
 
       # TODO(hans): Get this cleaned up, http://crbug.com/428099
       "-Wno-inconsistent-missing-override",
     ]
 
-    # NaCl's Clang compiler and Chrome's hermetic Clang compiler will almost
-    # always have different versions. Certain flags may not be recognized by
-    # one version or the other.
+    # Chrome's hermetic Clang compiler, NaCl's Clang compiler and Xcode's Clang
+    # compiler will almost always have different versions. Certain flags may not
+    # be recognized by one version or the other.
     if (!is_nacl) {
       # Flags NaCl (Clang 3.7) does not recognize.
       cflags += [
         # TODO(thakis): Enable this, crbug.com/507717
         "-Wno-shift-negative-value",
-
-        # TODO(thakis): https://crbug.com/604888
-        "-Wno-undefined-var-template",
       ]
 
       if (llvm_force_head_revision) {
         cflags += [
           # TODO(thakis): https://crbug.com/617318
           "-Wno-nonportable-include-path",
         ]
       }
     }
+    if (!is_nacl && !use_xcode_clang) {
+      # Flags NaCl (Clang 3.7) and Xcode 7.3 (Clang clang-703.0.31) do not
+      # recognize.
+      cflags += [
+        # TODO(thakis): https://crbug.com/604888
+        "-Wno-undefined-var-template",
+      ]
+    }
   }
 }
 
 # chromium_code ---------------------------------------------------------------
 #
 # Toggles between higher and lower warnings for code that is (or isn't)
 # part of Chromium.
 
 config("chromium_code") {
   if (is_win) {
@@ skipping 504 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }