getifaddrs missing more user friendly in CMake.

getifaddrs missing more user friendly in CMake.

Don't require the user to enable using the built-in BSD getifaddrs implementation on systems such as uclibc that lacks it manually.

Instead if getifaddrs doesn't exist, use the BSD one automatically, except if the user explicitly tells the user not to do this using WITHOUT_BUILTIN_GETIFADDRS (which will result in a compilation error, but at least with a nice error message explaining why).