Fix the preprocessing actions

[ghost]

Preprocessing actions shouldn't manually redirect the output to ${@} as dune already does this. This used to work, however it no longer works with dune >= 1.6.3 (see ocaml/dune#1811 for details).

We plan to make this a hard error in dune, to avoid other cases in the future.

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[ghost]

Do I need to sign a CLA for this patch? It is very small and trivial...

[AltGr]

You can ignore the CLA ;)
We should probably update the opam file to specify the minimum version of dune though ? (The dependency is still to jbuilder)

[Drup]

@AltGr We could just upgrade to dune on the way and ditch jbuilder couldn't we ?

[ghost]

Before this patch there should be a conflict with dune >= 1.6.3, but after this patch the conflict is no longer needed, i.e. the new code works with both old and new versions of dune. So we should update the opam file in the opam-repository, but it is not necessary to update the one in this repository.

Regarding moving to dune, we now have an automatic upgrader in dune master; running dune upgrade will convert the project to dune (except for the calls to jbuilder in the Makefile).

[AltGr]

great, thanks!

[Drup]

Let's merge, so that we can at least pin master! :)

[UnixJunkie]

There should be a bugfix release also.
Thanks!