DOS/Windows-specific code: ser-tcp.c

[Eli Zaretskii]

    * ser-tcp.c:

    #ifndef __CYGWIN32__
    #include <netinet/tcp.h>
    #endif

Is netinet/tcp.h header specific to Cygwin?  If so, this could be
replaced by a HAVE_NETINET_TCP_H macro set by Autoconf.