build: remove inconsistently used x-prefix comparisons
Given that these are already used inconsistently with `test` checks, it
makes sense to consolidate to not use the x-prefix comparisons at all.
In any case, it would be nice to think we no longer have to work around
a problem that existed in shells many, many years ago.
More info:
https://github.com/koalaman/shellcheck/wiki/SC2268
https://www.vidarholen.net/contents/blog/?p=1035
autoconf: fix getaddrinfo checking errors on mingw
`AC_CHECK_FUNCS` can not properly check `getaddrinfo` because this
function requires some special headers on mingw.
Using `AC_CHECK_DECL` can effectively solve this issue.
Same for
- getnameinfo
- getprotobynumber
- getservbyname
- inet_ntop
- inet_pton
