-std=c++11 is given on OCaml 5.0
================================

  $ make_dune_project 3.14

  $ cat >dune <<EOF
  > ;; will fail if :standard doesn't have -std=c++11 on OCaml >= 5.0
  > (executable
  >  (name cpp11)
  >  (foreign_stubs
  >   (language cxx)
  >   (names cpp11)
  >   (flags -std=c++98 :standard)))
  > EOF

  $ dune exec ./cpp11.exe
  Hi from C++11
