FreeRTOS/lwIP IPv6-only: LWS compile breaks because sockaddr_in is required even when LWIP_IPV4=0

[aksilicon]

When building libwebsockets for FreeRTOS + lwIP in IPv6-only mode (LWIP_IPV6=1, LWIP_IPV4=0) with LWS_WITH_IPV6=1, compilation fails because LWS code unconditionally depends on struct sockaddr_in and other IPv4-oriented symbols/types that lwIP omits when IPv4 is disabled. This appears to be a portability bug / assumption mismatch: compile-time IPv4 type dependency is enforced even for IPv6-only runtime use.

Environment:

Platform: FreeRTOS + lwIP
Toolchain: ARM GCC (embedded)
LWS: LWS_WITH_IPV6=1
lwIP: LWIP_IPV6=1, LWIP_IPV4=0

Observed Errors (examples):

include/libwebsockets/lws-adopt.h: field 'sa4' has incomplete type
lib/core/private-lib-core.h: field 'frt_pipe_si' has incomplete type

Impact: Prevents strict IPv6-only embedded builds and forces enabling IPv4 purely for compile-time type availability, even when application traffic is IPv6-only.

[lws-team]

We look after a lot of build combinations in CI but disabling ipv4 - you can disable all networking though - isn't even a selectable option in CMake atm. Bear in mind lws came from 2010...

I added an LWS_IPV4 cmake option that defaults-on (btw it's LWS_IPV6 which also defaults-on now) to main branch, and you can disable it to supposedly build without IPv4, it works on linux but I guess we might find things if you build it against a userland stack that doesn't have any ipv4 headers...