Dune does not correctly handle c_compiler using a shim

Expected Behavior

In a macos environment, it is normal to specify the c-compiler using a shim, for example : "xcrun -sdk macosx10.14 clang". I would expect Dune to use this completely, and not try to interpret the arguments to the shim as arguments to the compiler (c_flags)

Actual Behavior

Dune interprets "-sdk macosx10.14 clang" as arguments and when processing ".c" files, it gets passed to ocamlc as "-ccopt sdk -ccopt macosx10.14 -ccopt clang". This cause the compiler to fail as ocamlc passes these options to clang (ocamlc apparently picks up the compiler specification from the environment variables, and uses the full "xcrun -sdk macosx10.14 clang" internally)

Reproduction

• PR with a reproducing test:

1. Set "CC=xcrun -sdk macosx10.14 clang" in lib/ocaml/std-lib/Makefile.config
2. Compile a ocaml module having a c-stub (dune itself has fcntl_stubs.c which fails to compile on a mac with the above setting)

Specifications

• Version of dune (output of dune --version): 2.0.0
• Version of ocaml (output of ocamlc --version) : 4.06.1
• Operating system (distribution and version): macos Mohave 10.14.6

Additional information

• Link to gist with verbose output (run dune with the --verbose flag):

$ (cd _build/default/src/stdune && /usr/share/ocaml/bin/ocamlc.opt -g -I caml -ccopt -sdk -ccopt macosx10.14 -ccopt clang -ccopt -mmacosx-version-min=10.9 -ccopt -O2 -ccopt -pipe -ccopt -fno-common -ccopt -O2 -ccopt -fno-strict-aliasing -ccopt -fwrapv -ccopt -g -o fcntl_stubs.o fcntl_stubs.c)
clang: error: unknown argument: '-sdk'
clang: error: no such file or directory: 'macosx10.14'
clang: error: no such file or directory: 'clang'

Reproduce the failure using CC=xcrun -sdk macosx10.14 clang and the c-stub in lib/ocaml/std-lib/Makefile.config, then inspect how Dune handles that compiler setting. The change is done when compiling fcntl_stubs.c no longer forwards the shim's arguments as separate c_flags and the shown ocamlc command succeeds.