Generation of opam files with lang dune >= 1.11
-----------------------------------------------

  $ cat > dune-project <<EOF
  > (lang dune 1.11)
  > (name test)
  > (generate_opam_files true)
  > (package (name test))
  > EOF

  $ dune build @install
  $ cat test.opam
  # This file is generated by dune, edit dune-project instead
  opam-version: "2.0"
  depends: [
    "dune" {>= "1.11"}
  ]
  build: [
    ["dune" "subst"] {pinned}
    [
      "dune"
      "build"
      "-p"
      name
      "-j"
      jobs
      "@install"
      "@runtest" {with-test}
      "@doc" {with-doc}
    ]
  ]