Fix undefined HOST_NAME_MAX on Windows.

The Windows build was broken by commit
826ad11e419db90b66b3f76a90b54df021bb39fc 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.