compilation fixes

[hannesm]

best viewed commit by commit. this is supposed to achieve the following (by cherry-picking and polishing commits from a multitude of forks):

• use dune instead of ocamlbuild/topkg
• revert the "remove sexplib serialisers"
• use dune-configurator and bigarray-compat
• do not use -Wno-implicit-fallthrough, since this is a reasonable warning. instead, mark the case statements explicitly where a fallthrough is expected (use a C comment instead of annotation to make gcc<7 and clang not complain)
• fix "unused C function" warnings
• cross-compilation for freestanding and xen (by flattening the src/native directory and copying around)
• be more precise why warning -3 is in nocrypto_lwt_entropy

the freestanding and xen support is not yet properly tested (and may need some more work) works fine (a static_website_tls unikernel links, and the freestanding/hvt one also runs smoothly), I'll follow up on that. once this is good to go, the renaming could happen.

[emillon]

You can use Configurator.V1.Flags.write_sexp here.

[hannesm]

ok, I do not know how to do this. I am unable to find the documentation / API of Configurator.V1.Flags. this cfg.exe is used by src/dune (with a rule (rule (with-stdout-to c_flags.sexp (run ../config/cfg.exe)))) as well as xen/dune and freestanding/dune.

From https://dune.readthedocs.io/en/stable/quick-start.html#defining-a-library-with-c-stubs I gather that your proposed write_sexp takes two arguments, a file name and an OCaml value - but it is rather unclear to me where the output is written to, then (in terms of relative path).

[hannesm]

thanks for your reviews.

I force-pushed to add a lower bound of OCaml 4.07.0 -- and then removed the bigarray-compat commit (since this is only needed < 4.07.0 -- in 4.07.0+ we have Stdlib.Bigarray)

[hannesm]

hmm, now when I run dune utop, I get:

File "xen/dune", line 7, characters 12-28:
7 |  (libraries mirage-xen-posix)
                ^^^^^^^^^^^^^^^^
Error: Library "mirage-xen-posix" not found.
Hint: try: dune external-lib-deps --missing .utop/utop.exe

but in xen/dune the stanza optional is included -- any dune hints how to get dune utop to work in such a scenario (dune 2.1.3 is what I use)?

another question about CI: is it possible that it runs with optional dependencies? (in case the answer is no (what I expect), I guess an appropriate solution is to have separate packages (though I'm not sure how this would work with the META.in, but I can try to figure that out).

[talex5]

another question about CI: is it possible that it runs with optional dependencies? (in case the answer is no (what I expect), I guess an appropriate solution is to have separate packages (though I'm not sure how this would work with the META.in, but I can try to figure that out).

Yeah, separate packages would be great!

If you need to force optional deps to be installed in CI as a quick fix though you could always add a dummy opam package that depends on all the optional stuff. Or you could make the optional dependencies required for tests.

[hannesm]

@talex5 thanks for your quick and valuable response. Since I'm not eager to accumulate hacks in this repository, but will subsequently revise the package layout (not in this PR).