OCaml 4.03 compatibility

[Khady]

I am not sure about the fix. I followed the suggestion of the compiler. But the test is passing.

[whitequark]

This breaks it on 4.02. Please add cppo like in this commit: ocaml-ppx/ppx_deriving_yojson@9417696

The files you need are opam, pkg/build.ml, myocamlbuild.ml, and this hack: ocaml-ppx/ppx_deriving_yojson@9417696#diff-7fa1cddad622d88b43f3838f86e70552R3.

[whitequark]

And I would also appreciate if you ported this commit over as well: ocaml-ppx/ppx_deriving_yojson@3922531

[Khady]

Thanks for the quick review. I will have a look at your links.

[Khady]

PR updated. Here is the log of a travis build: https://travis-ci.org/Khady/ppx_getenv/builds/127526467

[whitequark]

Thanks!

[gasche]

Would it be possible to do a ppx_getenv release with this fix? opam-builder currently reports that no ppx_getenv version works under 4.03 and 4.04 -- there may also be 4.04-specific issue but the automated builder fails on this incompatibility.

[whitequark]

@gasche Wish granted ocaml/opam-repository#7197

[gasche]

Thanks! I may update the metadata of previous versions to mark availability as < 4.03, and I will let you know here if the automated builder gives further 4.04-specific issues.