message: Include gtest headers with -isystem (or similar)
diff: @@ -140,13 +140,8 @@ target_link_libraries(unittests libgtest libgtest_main ${CMAKE_THREAD_LIBS_INIT}
  # Set gtest include dirs as target properties
  # This way cmake does not complain about not (yet) existing include dirs
  # at first invocation
  -
  -# Extend already present include dirs list, to retain standard cmake behavior
  -# (inheritance, etc.)
  -get_property(ORIGINAL_INCLUDE_DIRS TARGET unittests PROPERTY INCLUDE_DIRECTORIES)
  -
  -set_target_properties(unittests PROPERTIES
  - "INCLUDE_DIRECTORIES" "${ORIGINAL_INCLUDE_DIRS};${GTEST_INCLUDE_DIR}"
  +set_target_properties(unittests PROPERTIES APPEND_STRING PROPERTY
  + COMPILE_FLAGS " ${CMAKE_INCLUDE_SYSTEM_FLAG_CXX} ${GTEST_INCLUDE_DIR}"
  )
  # Allow multi-line comments, some unit tests have verbose documentation with extra '/'s and '\'s
  