Incorrect passing of linker options from pkg-config to ocamlmklib

This issue is probably similar to #2977, it just adds that the problem may easily arise when using pkg-config.

Expected Behavior

I follow the documentation to compile code with foreign C libraries using pkg-config for options.

Actual Behavior

ocamlmklib emits warnings about link options returned by pkg-config, such as -Wl,--export-dynamic or -fopenmp. I'm not sure about the actual consequences of omitting these options, but I'm guessing the problem I encounter is recurrent when using pkg-config.

Reproduction

pveber/ocaml-r#14 provides more details and a way to reproduce the problem.

Specifications

• Version of dune (output of dune --version):

2.7.1

• Version of ocaml (output of ocamlc --version)

4.11.1

• Operating system (distribution and version):
  Ubuntu focal

Start with the ocamlmklib handling described in the issue and reproduce the warning using the pveber/ocaml-r#14 example with pkg-config. Compare the linker options returned by pkg-config, including -Wl,--export-dynamic and -fopenmp, with the options ocamlmklib accepts. Done means the options are passed correctly without warnings and the reproduction works on the stated Dune and OCaml versions.