On why wsi.mli is symlinked from the root to wsi/_platform_/wsi.mli

    $ touch {a.ml,d/a.mli}
    $ (cd d; ocamlc -c a.mli)
    $ ls
    a.ml  d
    $ ls d
    a.cmi  a.mli
    $ ocamlc -c -I d a.ml
    $ ls
    a.cmi  a.cmo  a.ml  d

In other words - (it appears) ocamlc only expects mli to be in the
same directory as the module being compiled.
