pass clangcl compile options to ignore warnings in gflags.cc

R=ajm@chromium.org, ajm@google.com
BUG=libyuv:513,webrtc:760

Review URL: https://codereview.chromium.org/1427643003 .

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1526
+Version: 1527
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1526
+#define LIBYUV_VERSION 1527
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT

third_party/gflags/gflags.gyp

@@ -63,6 +63,18 @@
         }],
         # TODO(andrew): Look into fixing this warning upstream:
         # http://code.google.com/p/webrtc/issues/detail?id=760
+        ['OS=="win" and clang==1', {
+          'msvs_settings': {
+            'VCCLCompilerTool': {
+              'AdditionalOptions!': [
+                '-Wheader-hygiene',  # Suppress warning about using namespace.
+              ],
+              'AdditionalOptions': [
+                '-Wno-unused-local-typedef',  # Suppress unused private typedef.
+              ],
+            },
+          },
+        }],
         ['clang==1', {
           'cflags!': ['-Wheader-hygiene',],
           'xcode_settings': {