message: ruby: GCC 10 doesn't like some of ruby.h's choices
  No way to deal with them other than flagging as system.
diff: find_package(Ruby)
  if (RUBY_FOUND)
  add_plugin(Ruby "Ruby.cpp")
  - target_include_directories(Ruby PRIVATE "${RUBY_INCLUDE_DIRS}")
  + target_include_directories(Ruby SYSTEM PRIVATE "${RUBY_INCLUDE_DIRS}")
  target_link_libraries(Ruby ${RUBY_LIBRARY})
  endif (RUBY_FOUND)
  