GN: detect is_clang, use it to switch Clang to warning blacklist.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2599

Change-Id: I09c3487adfeb26a6fb07e1939cb927c5d7de3107
Reviewed-on: https://skia-review.googlesource.com/2599
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gn/BUILD.gn

@@ -24,6 +24,13 @@ declare_args() {
   cc_wrapper = ""
 }
 
+is_clang = exec_script("is_clang.py",
+                       [
+                         cc,
+                         cxx,
+                       ],
+                       "value")
+
 config("default") {
   asmflags = []
   cflags = []
@@ -70,6 +77,73 @@ config("default") {
 
       "-Wnon-virtual-dtor",
     ]
+    if (is_clang) {
+      cflags += [
+        "-Weverything",
+        "-Wno-unknown-warning-option",  # Let older Clangs ignore newer Clangs' warnings.
+      ]
+
+      # High priority to fix!
+      cflags += [
+        "-Wno-comma",
+        "-Wno-conditional-uninitialized",
+        "-Wno-covered-switch-default",
+        "-Wno-deprecated",
+        "-Wno-format-nonliteral",
+        "-Wno-shadow",
+        "-Wno-shift-sign-overflow",
+        "-Wno-undefined-func-template",
+        "-Wno-undefined-reinterpret-cast",
+      ]
+
+      cflags += [
+        "-Wno-cast-align",
+        "-Wno-class-varargs",
+        "-Wno-conversion",
+        "-Wno-disabled-macro-expansion",
+        "-Wno-documentation",
+        "-Wno-documentation-unknown-command",
+        "-Wno-double-promotion",
+        "-Wno-exit-time-destructors",  # TODO: OK outside libskia
+        "-Wno-extra-semi",
+        "-Wno-float-conversion",
+        "-Wno-float-equal",
+        "-Wno-global-constructors",  # TODO: OK outside libskia
+        "-Wno-gnu-anonymous-struct",
+        "-Wno-gnu-zero-variadic-macro-arguments",
+        "-Wno-missing-prototypes",
+        "-Wno-missing-variable-declarations",
+        "-Wno-nested-anon-types",
+        "-Wno-newline-eof",
+        "-Wno-pedantic",
+        "-Wno-reserved-id-macro",
+        "-Wno-sign-conversion",
+        "-Wno-switch-enum",
+        "-Wno-undef",
+        "-Wno-unreachable-code",
+        "-Wno-unreachable-code-break",
+        "-Wno-unreachable-code-return",
+        "-Wno-unused-macros",
+        "-Wno-unused-member-function",
+      ]
+      cflags_cc += [
+        "-Wno-abstract-vbase-init",
+        "-Wno-range-loop-analysis",
+        "-Wno-weak-vtables",
+      ]
+
+      # We are unlikely to want to fix these.
+      cflags += [
+        "-Wno-implicit-fallthrough",
+        "-Wno-missing-noreturn",
+        "-Wno-old-style-cast",
+        "-Wno-padded",
+      ]
+      cflags_cc += [
+        "-Wno-c++98-compat",
+        "-Wno-c++98-compat-pedantic",
+      ]
+    }
   }
 
   if (current_cpu == "arm") {

gn/is_clang.py

@@ -0,0 +1,17 @@
+#!/usr/bin/env python
+#
+# Copyright 2016 Google Inc.
+#
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import subprocess
+import sys
+cc,cxx = sys.argv[1:3]
+
+if ('clang' in subprocess.check_output([cc,  '--version']) and
+    'clang' in subprocess.check_output([cxx, '--version'])):
+  print 'true'
+else:
+  print 'false'
+