Fix undefined HOST_NAME_MAX on Windows

The Windows build was broken by commit 826ad11e419db90b66b3f76a90b54df021bb39fc (by @wkennington) which introduced a dependency on the `HOST_NAME_MAX` macro, which is not defined on Windows. According to MSDN for `gethostname()`, the maximum length of the returned string is 256 bytes (including the terminating null byte), so let's use that as a fallback.