Fix MSVC support for stub DLLs

[dra27]

JBuilder issue: it just needs the ###### filled in

On mingw, and other Unix-like compiler toolchains, shared libraries are linked with -lfoo searching for libfoo.a. For the Microsoft toolchain, this convention doesn't apply. So for mingw I have:

   (c_library_flags (:standard -lgdi32 -luser32 -lshell32))

and for MSVC I translate this to:

   (c_library_flags (:standard gdi32.lib user32.lib shell32.lib))

However, jbuilder passes those libraries in the wrong way to various subcommands resulting in this output:

  ocamlmklib src/core/dllopam_core_stubs.dll,src/core/libopam_core_stubs.lib
user32.lib(USER32.dll) : warning LNK4006: __NULL_IMPORT_DESCRIPTOR already defined in gdi32.lib(GDI32.dll); second definition ignored
user32.lib(USER32.dll) : warning LNK4221: no public symbols found; archive member will be inaccessible
shell32.lib(SHELL32.dll) : warning LNK4006: __NULL_IMPORT_DESCRIPTOR already defined in gdi32.lib(GDI32.dll); second definition ignored
shell32.lib(SHELL32.dll) : warning LNK4221: no public symbols found; archive member will be inaccessible
advapi32.lib(ADVAPI32.dll) : warning LNK4006: __NULL_IMPORT_DESCRIPTOR already defined in gdi32.lib(GDI32.dll); second definition ignored
advapi32.lib(ADVAPI32.dll) : warning LNK4221: no public symbols found; archive member will be inaccessible

The warning is innocuous, but irritating. I think the correct way is to pass this option to ocamlmklib using -ldopt (I think it's OK that it's then only used to create the .dll and not also embedded in the .cma/.cmxa because I think that flexlink already deals with that further down the line). However, -ldopt can't be used in c_library_flags because it will also be passed to ocamlopt which will interpret it as requiring libdopt!

I think that ocamlmklib should handle this difference itself (see ocaml/ocaml#1189), but even if ocamlmklib is changed in 4.06, jbuilder will need to handle older versions so I think some kind of support is needed here. My preference would be for jbuilder to recognise -lfoo and automatically translate it when passing it to tools on an msvc port (possibly except if libfoo.lib actually exists, but I think this will be quite rare). See also ocaml/flexdll#32

[ghost]

Would it worked if jbuilder wrapped all options in c_library_flags with -ldopt <opt> when passing them to ocamlmklib? It already wraps them with -cclib when passing them to ocamlopt.

[ghost]

Technically, I'm not sure whether ocamlmklib does something special with -l... and -L... options or if it just passes them to the underlying command.

[ghost]

I looked a bit more into this, wrapping all options with -ldopt will not work well as ocamlmklib rearranges -L and -l options in a specific order. I added a special case for msvc, I think it should fix this issue. Once ocaml/ocaml#1189 is merged we can disable the special case for OCaml > 4.06.

@dra27 can you check that it does fixes the issue?