thfrwn reports that OpenBSD (almost) works as-is, except that its
Python `sys.platform` is not in the allowed values list. Add it to the
list. By happy accident, unhandled `host_platform` is treated as
`'linux'` and `'linux'` produces the right result for OpenBSD.
Requested-by: thfrwn <https://github.com/dxx-rebirth/dxx-rebirth/pull/359>