libcurl: use c-ares if available

By default libcurl uses the C library's DNS resolver which is
synchronous, even if an application is using libcurl's non-blocking mode
of operation.

Configure libcurl to use c-ares if it is selected so that it can resolve
addresses asynchronously if required.

[Peter: explicitly disable c-ares support if not enabled]
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>