findlib warnings for duplicated interface lib/ocaml and lib/ocaml/compiler-libs

[bsansouci]

This is a problem in OCaml 4.02.3. I think there's a fix (kinda thought we fixed this).

~/Desktop/tmp/example2 master*
❯ time ~/Desktop/bucklescript/lib/bsb -backend bytecode
ninja: Entering directory `lib/bs/bytecode'
[4/4] Building src/test.mlast.d
[1/3] Building src/test.cmo
findlib: [WARNING] Interface topdirs.cmi occurs in several directories: /Users/benjamin/.opam/4.02.3+buckle-master/lib/ocaml/compiler-libs, /Users/benjamin/.opam/4.02.3+buckle-master/lib/ocaml
[2/3] Building src/hello.cmo
findlib: [WARNING] Interface topdirs.cmi occurs in several directories: /Users/benjamin/.opam/4.02.3+buckle-master/lib/ocaml/compiler-libs, /Users/benjamin/.opam/4.02.3+buckle-master/lib/ocaml
[3/3] Building hello.byte
findlib: [WARNING] Interface topdirs.cmi occurs in several directories: /Users/benjamin/.opam/4.02.3+buckle-master/lib/ocaml/compiler-libs, /Users/benjamin/.opam/4.02.3+buckle-master/lib/ocaml
~/Desktop/bucklescript/lib/bsb -backend bytecode  0.51s user 0.23s system 88% cpu 0.829 total

cc @Schmavery