This test demonstrates that -ppx is no more missing when two stanzas are
in the same dune file, but require different ppx specifications

  $ ocamlc_where="$(ocamlc -where)"
  $ export BUILD_PATH_PREFIX_MAP="/OCAMLC_WHERE=$ocamlc_where:$BUILD_PATH_PREFIX_MAP"

  $ dune build @all --profile release
  $ dune ocaml merlin dump-config $PWD
  Usesppx1: _build/default/usesppx1
  ((INDEX $TESTCASE_ROOT/_build/default/.usesppx2.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/.usesppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx1/.ppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx2/.ppx2.objs/cctx.ocaml-index)
   (STDLIB /OCAMLC_WHERE)
   (SOURCE_ROOT $TESTCASE_ROOT)
   (EXCLUDE_QUERY_DIR)
   (B $TESTCASE_ROOT/_build/default/.usesppx1.objs/byte)
   (S $TESTCASE_ROOT)
   (FLG (-ppx "$TESTCASE_ROOT/_build/default/.ppx/c152d6ca3c7e1d83471ffdf48bf729ae/ppx.exe --as-ppx --cookie 'library-name="usesppx1"'"))
   (FLG (-w -40 -g))
   (UNIT_NAME usesppx1))
  Usesppx1: _build/default/usesppx1.ml-gen
  ((INDEX $TESTCASE_ROOT/_build/default/.usesppx2.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/.usesppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx1/.ppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx2/.ppx2.objs/cctx.ocaml-index)
   (STDLIB /OCAMLC_WHERE)
   (SOURCE_ROOT $TESTCASE_ROOT)
   (EXCLUDE_QUERY_DIR)
   (B $TESTCASE_ROOT/_build/default/.usesppx1.objs/byte)
   (S $TESTCASE_ROOT)
   (FLG (-ppx "$TESTCASE_ROOT/_build/default/.ppx/c152d6ca3c7e1d83471ffdf48bf729ae/ppx.exe --as-ppx --cookie 'library-name="usesppx1"'"))
   (FLG (-w -40 -g))
   (UNIT_NAME usesppx1))
  Usesppx2: _build/default/usesppx2
  ((INDEX $TESTCASE_ROOT/_build/default/.usesppx2.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/.usesppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx1/.ppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx2/.ppx2.objs/cctx.ocaml-index)
   (STDLIB /OCAMLC_WHERE)
   (SOURCE_ROOT $TESTCASE_ROOT)
   (EXCLUDE_QUERY_DIR)
   (B $TESTCASE_ROOT/_build/default/.usesppx2.objs/byte)
   (S $TESTCASE_ROOT)
   (FLG (-ppx "$TESTCASE_ROOT/_build/default/.ppx/d7394c27c5e0f7ad7ab1110d6b092c05/ppx.exe --as-ppx --cookie 'library-name="usesppx2"'"))
   (FLG (-w -40 -g))
   (UNIT_NAME usesppx2))
  Usesppx2: _build/default/usesppx2.ml-gen
  ((INDEX $TESTCASE_ROOT/_build/default/.usesppx2.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/.usesppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx1/.ppx1.objs/cctx.ocaml-index)
   (INDEX $TESTCASE_ROOT/_build/default/ppx2/.ppx2.objs/cctx.ocaml-index)
   (STDLIB /OCAMLC_WHERE)
   (SOURCE_ROOT $TESTCASE_ROOT)
   (EXCLUDE_QUERY_DIR)
   (B $TESTCASE_ROOT/_build/default/.usesppx2.objs/byte)
   (S $TESTCASE_ROOT)
   (FLG (-ppx "$TESTCASE_ROOT/_build/default/.ppx/d7394c27c5e0f7ad7ab1110d6b092c05/ppx.exe --as-ppx --cookie 'library-name="usesppx2"'"))
   (FLG (-w -40 -g))
   (UNIT_NAME usesppx2))
