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

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

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
```