quagga: fix non-IPv6 build failures

The Zebra daemon now requires IPv6 support so disable it for non-IPv6
toolchains and add a comment mentioning it.
The BABEL protocol/daemon also requires IPv6 so just enable it when it's
available.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>