LWIP_ALTCP macro

LWIP_ALTCP==1: enable the altcp API. altcp is an abstraction layer that prevents applications linking against the tcp.h functions but provides the same functionality. It is used to e.g. add SSL/TLS or proxy-connect support to an application written for the tcp callback API without that application knowing the protocol details. With LWIP_ALTCP==0, applications written against the altcp API can still be compiled but are directly linked against the tcp.h callback API and then cannot use layered protocols. See altcp_api