cmake: enable IPv6 by default if available

ENABLE_IPV6 depends on HAVE_GETADDRINFO or you will get a
Curl_getaddrinfo_ex error. Enable IPv6 by default, disabling it if
struct sockaddr_in6 is not found in netinet/in.h.

Note that HAVE_GETADDRINFO_THREADSAFE is still not set as it needs more
platform checks even though POSIX requires a thread-safe getaddrinfo.

Verified on Arch Linux x86_64 with glibc 2.20-2 and Linux 3.16-rc7.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>