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

gn/is_clang.py

Index: gn/is_clang.py
diff --git a/gn/is_clang.py b/gn/is_clang.py
new file mode 100755
index 0000000000000000000000000000000000000000..99173a004c521f8073f4c6faf2cb52e89120967f
--- /dev/null
+++ b/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'
+