tests: fix enum/int confusion (Intel C), fix autotools `CFLAGS` for `servers`

tests: fix enum/int confusion (Intel C), fix autotools `CFLAGS` for `servers`

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:40:338

Closes #16314