Fix Makefile.rules on OpenBSD

[abbysmal]

Hello,

When compiling on OpenBSD I encountered this error

ocamlfind ocamlmklib -o _build/ctypes-foreign-base_stubs _build/src/ctypes-foreign-base/dl_stubs.o _build/src/ctypes-foreign-base/ffi_call_stubs.o _build/src/ctypes-foreign-base/ffi_type_stubs.o -L/usr/local/lib -pthread -lffi
ocamlfind: unknown option '-pthread'.

That seems related to a line in Makefile.rules.
I fixed that a few weeks ago but can't remember much about the fix itself, I'm not sure I understand what's going on.
I tested it and it looks like it works on Linux and OpenBSD at least.
Do you have any idea on what might be the issue ?

patch attached to fix the problem.

[yallop]

I don't think that the proposed fix is quite what's needed -- at least, I don't see a useful interpretation of % here.

Perhaps the issue is that the linker flags are being passed directly to ocamlmklib, but should instead be prefixed with -cclib. Does the following work for you on OpenBSD?

OCAMLMKLIB_FLAGS=$($(PROJECT).link_flags:%=-cclib %)

[abbysmal]

Seems like it works! Thank you.
May I close the PR and let you add the change to the repo ? Or should I change my PR to reflect to change ?

[yallop]

Thanks for testing.

If you push the change to this branch I'll merge it in.

[xguerin]

Just a quick up to also mention that I have the same problem on OpenBSD. To bad for the revert. Any idea how to approach it ? I can look into it if necessary.

[yallop]

I'm not sure what the best approach is, and don't have an OpenBSD machine available to investigate, but a patch would be very welcome.

[xguerin]

Alright, so here is the deal:

• it looks like ocamlfind ocamlmklib calls on ld directly
• on OpenBSD, pkg-config is used to get the link flags, which include -pthread
• when used verbatim with ld, these options fail as -pthread is not a valid flag for OpenBSD ld

I am not quite sure what to think of it yet. I asked the Internet for similar issues, but rare are the occasions where ld is used directly with pkg-config.

Also, given that I am uncertain of the scope of pkg-config, I am not certain on whom to call fool yet. Indeed, the pkg-config --libs line would work perfectly fine with gcc or clang.

I'll shoot an e-mail on the OpenBSD mailing list and see if they have a a heuristic for that situation.

[yallop]

@xguerin: Thanks for the details. Could you check whether #454 solves the issue for you on OpenBSD?

[xguerin]

@yallop Yes, I can attest that that #454 fixes the compilation problem with OpenBSD.

[yallop]

Excellent; thanks, @xguerin!