configure.ac: fix failing IPv6 check due to missing return type

[bobrik]

Fixing this warning escalated to an error, resuting in no IPv6 support:

configure.sh:7679: checking whether to enable ipv6
configure.sh:7718: clang -o conftest -g -O2 -DHAVE_CONFIG_H -Wall -W   conftest.c  >&5
conftest.c:73:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
main()
^
int
1 error generated.
configure.sh:7718: $? = 1
configure.sh: program exited with status 1

[bobrik]

It was a warning on clang-15, but it is an error on clang-16:

• https://github.com/NixOS/nixpkgs/pull/278184

[bobrik]

A similar thing happened to curl: https://github.com/curl/curl/pull/12607

[bobrik]

I'm happy to pull the curl's version verbatim if you think that's better.

[tridge]

@bobrik looks good, thanks!