message: cmake: flag gtest/gmock as system include
  This way, we don't get warnings for gtest/gmock.
diff: @@ -163,13 +163,10 @@ if(NOT IOS AND NOT ANDROID)
  enable_testing()
  add_subdirectory(libs/gtest EXCLUDE_FROM_ALL)
  - include_directories(libs/gtest/googletest/include)
  - include_directories(libs/gtest/googlemock/include)
  - if(NOT MSVC)
  - # gtest has these warnings
  - add_definitions("-Wno-sign-compare")
  - endif()
  + # SYSTEM because we don't want warnings for gtest headers.
  + include_directories(SYSTEM libs/gtest/googletest/include)
  + include_directories(SYSTEM libs/gtest/googlemock/include)
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DTESTING")
  