Pull new version of protobuf sources.

third_party/protobuf/appveyor.bat

Index: third_party/protobuf/appveyor.bat
diff --git a/third_party/protobuf/appveyor.bat b/third_party/protobuf/appveyor.bat
new file mode 100644
index 0000000000000000000000000000000000000000..356a13f86bf5b20534810c4a574cefd3f6125131
--- /dev/null
+++ b/third_party/protobuf/appveyor.bat
@@ -0,0 +1,29 @@
+setlocal
+
+IF %language%==cpp GOTO build_cpp
+IF %language%==csharp GOTO build_csharp
+
+echo Unsupported language %language%. Exiting.
+goto :error
+
+:build_cpp
+echo Building C++
+mkdir build_msvc
+cd build_msvc
+cmake -G "%generator%" -DBUILD_SHARED_LIBS=%BUILD_DLL% ../cmake
+msbuild protobuf.sln /p:Platform=%vcplatform% /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" || goto error
+cd %configuration%
+tests.exe || goto error
+goto :EOF
+
+:build_csharp
+echo Building C#
+cd csharp\src
+nuget restore
+msbuild Google.Protobuf.sln /p:Platform="Any CPU" /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" || goto error
+nunit-console Google.Protobuf.Test\bin\%configuration%\Google.Protobuf.Test.dll || goto error
+goto :EOF
+
+:error
+echo Failed!
+EXIT /b %ERRORLEVEL%