pass clangcl compile options to ignore warnings in gflags.cc

third_party/gflags/gflags.gyp

Index: third_party/gflags/gflags.gyp
diff --git a/third_party/gflags/gflags.gyp b/third_party/gflags/gflags.gyp
index 9c11e75a44a128121a04358351c8699834a36cfb..57567fa3ba0f6e52861639b79021fa9bb76a1691 100644
--- a/third_party/gflags/gflags.gyp
+++ b/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': {