slirp: Avoid redefining MAX_TCPOPTLEN

MAX_TCPOPTLEN is being defined as 32. Darwin already has it as 40,
causing a warning. The value is only used to declare an array,
into which currently 4 bytes are written at most.

Therefore always override MAX_TCPOPTLEN for now.

Suggested-by: Jan Kiszka <jan.kiszka@web.de>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
