mingw support

CockroachDB would like to support Windows, but most of our infrastructure and in-house knowledge is in Unix, not Windows. Ideally, we would like to compile RocksDB for windows using mingw, but that seems quite unsupported at the moment.

Concretely, it seems that the `make`-based build system doesn't support `OS_WIN`, and the `cmake`-based build system doesn't support anything other than `MSVC` on windows.

I sent a preliminary patch (https://github.com/facebook/rocksdb/pull/1910), but next steps seems to require build-system support, and that's not an area I know much about.

cc @yuslepukhin who seems to be the windows guy.