message: Add gtest include dir with -isystem
diff: @@ -143,11 +143,17 @@ target_link_libraries(unittests libgtest libgtest_main ${CMAKE_THREAD_LIBS_INIT}
  # Extend already present include dirs list, to retain standard cmake behavior
  # (inheritance, etc.)
  +if(MSVC)
  +# MSVC has no -isystem equivalent
  get_property(ORIGINAL_INCLUDE_DIRS TARGET unittests PROPERTY INCLUDE_DIRECTORIES)
  -
  set_target_properties(unittests PROPERTIES
  "INCLUDE_DIRECTORIES" "${ORIGINAL_INCLUDE_DIRS};${GTEST_INCLUDE_DIR}"
  )
  +else()
  +# Set gtest includes with -isystem
  +set_target_properties(unittests PROPERTIES
  + APPEND_STRING PROPERTY COMPILE_FLAGS " ${CMAKE_INCLUDE_SYSTEM_FLAG_CXX} ${GTEST_INCLUDE_DIR}")
  +endif()
  # Allow multi-line comments, some unit tests have verbose documentation with extra '/'s and '\'s
  if(CMAKE_COMPILER_IS_GNUCXX)
  