Okay, so it looks like we don't have any prior art to conditionally omit constants from _socket based on OS level, and nothing in the setsockopt() doc suggests that it may fail like this.

So we either need to explicitly exclude this symbol on Windows (at least until we drop support for pre-Windows 10 versions) or silently ignore setsockopt errors for future good arguments. I'm inclined to do the former - other opinions?