message: MSVC does not have -isystem or an equivalent
diff: @@ -137,12 +137,20 @@ visionaray_add_executable(unittests
  target_link_libraries(unittests libgtest libgtest_main ${CMAKE_THREAD_LIBS_INIT})
  -# Set gtest include dirs as target properties
  +# Set gtest include dirs via target properties
  # This way cmake does not complain about not (yet) existing include dirs
  # at first invocation
  +if(MSVC)
  + # MSVC does not have -isystem or an equivalent
  + set_target_properties(unittests PROPERTIES APPEND_STRING PROPERTY
  + INCLUDE_DIRECTORIES " ${GTEST_INCLUDE_DIR}"
  + )
  +else()
  + # Unix: include with -isystem to suppress warnings from gtest.h
  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)
  