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 60e136a5d9f5ec57f36d06ccc6b1612251439153..364f51b35854d99865a2cf4ff0f39241be389a4b 100644
--- a/third_party/gflags/gflags.gyp
+++ b/third_party/gflags/gflags.gyp
@@ -63,6 +63,16 @@
           ],
           # Suppress warnings about WIN32_LEAN_AND_MEAN and size_t truncation.
           'msvs_disabled_warnings': [4005, 4267],
+          'msvs_settings': {

[kjellander_webrtc, 2015/10/28 12:09:01]

After looking at the trybot failure for regular (non-Clang) build on Win it
seems you have to put this into a condition like 'OS=="win" and clang==1': 

[302/2847] LIB obj\third_party\libjpeg_turbo\libjpeg.lib
FAILED: ninja -t msvc -e environment.x64 -- E:\b\build\goma/gomacc
"E:\b\depot_tools\win_toolchain\vs2013_files\VC\bin\amd64\cl.exe" /nologo
/showIncludes /FC @obj\third_party\gflags\src\windows\gflags.port.obj.rsp /c
..\..\third_party\gflags\src\windows\port.cc
/Foobj\third_party\gflags\src\windows\gflags.port.obj
/Fdobj\third_party\gflags\gflags.cc.pdb 
cl : Command line error D8021 : invalid numeric argument
'/Wno-unused-local-typedef'

+            'VCCLCompilerTool': {
+              'AdditionalOptions!': [
+                '-Wheader-hygiene',  # Suppress warning about using namespace.
+              ],
+              'AdditionalOptions': [
+                '-Wno-unused-local-typedef',  # Suppress unused private typedef.
+              ],
+            },
+          },
         }],
         # TODO(andrew): Look into fixing this warning upstream:
         # http://code.google.com/p/webrtc/issues/detail?id=760