CMake: Reduce MSVC warning level

c++/CMakeLists.txt

@@ -33,7 +33,7 @@ else()
 endif()
 
 if(MSVC)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Wall ${CAPNP_LITE_FLAG}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W3 ${CAPNP_LITE_FLAG}")
   # Statically link with the runtime libraries.
   # TODO: This should depend on whether or not Cap'n Proto is being built as a static or shared library.
   #       When building the tests, this needs to match with how gtest is built as well.