cmake: Fix configuration for Illumos/SmartOS.

[hebasto]

This PR fixes a buggy implementation of the getifaddrs/freeifaddrs symbols checking in the ifaddrs.h header.

On illumos/SunOS:

• on the master branch:

$ cmake -B build
<snip>
-- Looking for freeifaddrs
-- Looking for freeifaddrs - not found
-- Looking for getifaddrs
-- Looking for getifaddrs - not found
<snip>

• with this PR:

$ cmake -B build
<snip>
-- Looking for C++ include sys/types.h
-- Looking for C++ include sys/types.h - found
-- Looking for C++ include ifaddrs.h
-- Looking for C++ include ifaddrs.h - found
-- Performing Test IFADDR_LINKS_WITHOUT_LIBSOCKET
-- Performing Test IFADDR_LINKS_WITHOUT_LIBSOCKET - Failed
-- Performing Test IFADDR_NEEDS_LINK_TO_LIBSOCKET
-- Performing Test IFADDR_NEEDS_LINK_TO_LIBSOCKET - Success
<snip>

[TheCharlatan]

lgtm

Seems closer to what we are doing in autotools and seems to only set the symbols if the check has passed now.