By dropping the unused enum wrappers for `AF_*` macros.
Also fix `./configure` to apply `--enable-werror` options to
`tests/servers`, to catch this next time.
Seen with Intel C compiler:
```
socksd.c(184): warning #188: enumerated type mixed with another type
socksd.c(881): warning #188: enumerated type mixed with another type
[...]
sws.c(76): warning #188: enumerated type mixed with another type
sws.c(229): warning #188: enumerated type mixed with another type
[...]
```
Ref: https://github.com/curl/curl/actions/runs/13296520425/job/37129676921#step:40:338Closes#16314