msvc linker response file should be UTF-16 encoded when possible

[nojb]

Currently we use a response file to pass arguments to the msvc linker when the command line is too long.

Supported encodings for the response file are ANSI (currently used) and UTF-16: see https://msdn.microsoft.com/en-us/library/xwy0e8f2(v=vs.140).aspx.

In order to make flexlink work with Unicode (cf ocaml/ocaml#1200) we need to use UTF-16 (here I am thinking about the situation when our OCaml strings are UTF-8). I made a quick proof of concept with a handwritten UTF-8 decoder and it fixed the problems I was seeing.

Ideally this functionality would be enabled as soon as flexlink is compiled with a Unicode-enabled ocamlc, but am not sure if this is easy to do.

Ideas ?

[nojb]

This has been addressed in #34.