fix for ocamlbuild's pack+ocamlfind handling

[gasche]

This PR fixes the issue that causes the build failure discussed in

ocaml/opam-repository#11628 (comment)

and

#143

I have another branch v0.5.4-ocamlbuild-pack against the v0.5.4 tag, where the _tags changes are slightly different.

[hannesm]

hmm, IIUC, this makes ppx_sexp_conv (which depends on ocaml-migrate-parsetree etc.) a runtime dependency of nocrypto!? my intuition is that these should better stay build-time only dependencies, is there another way?

[gasche]

ppx_sexp_conv (maybe only recent versions?) has a runtime library that I suppose they use for hygiene (to alias things from the global environment that code generators use), so I don't think we can avoid having it as a runtime dependency. There is a more precise ppx_sexp_conv.runtime-lib ocamlfind package, but I don't think using it improves things noticeably, and it breaks on older versions of the library.

[copy]

Note: the package ppx_sexp_conv.runtime-lib would suffice, but it is
only available as such under recent ppx_sexp_conv versions, so its
explicit use would make the build description (needlessly)
incompatible with older ppx_sexp_conv versions.

Wouldn't ppx_sexp_conv also pull in ppx_sexp_conv.expander, which in turn contains the whole build-time ppx machinery? sexplib v0.10 already included the runtime library, so I'd be in favour of constraining to sexplib>=v0.10 and using ppx_sexp_conv.runtime-lib rather than using ppx_sexp_conv.

[gasche]

You should test this, but I think from the META file that the .expander dependency should only exists under the ppx_driver predicate.

[copy]

You should test this, but I think from the META file that the .expander dependency should only exists under the ppx_driver predicate.

Ah, indeed it doesn't get linked. (tested by adding module X = Ppx_sexp_conv_expander in the code and making sure it doesn't compile).

[copy]

@gasche In the v0.5.4-ocamlbuild-pack branch the ocamlfind_and_pack commit is reverted. Is this due to the fix already being in ocamlbuild? I re-applied the commit and it compiles fine. I'm about to submit these commits as patch files to opam-repository, including the reverted commit.