Dropped MSVC support

[jonahbeckford]

There was commit 3d2ce1b that supported the MSVC compiler. But it got removed.

As a temporary hack, for Windows users of Diskuv OCaml (DKML) I've patched this package in the DKML opam repository.

[jonahbeckford]

One related MSVC note. During the build I get:

cl : Command line warning D9002 : ignoring unknown option '-std=c11'
cl : Command line warning D9002 : ignoring unknown option '-fPIC'

I am using MSVC 2019, where it just warns but does not fail the compilation. Often newer versions of MSVC will fail when a warning has been repeated long enough.