Cygwin is POSIX, don't say it isn't

More to the point, Cygwin is a POSIX API.  In our library, the use of
a POSIX API is marked by defining the macro OPENSSL_SYS_UNIX.
Therefore, that macro shouldn't be undefined when building for Cygwin.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5060)