configure: Detect and don't try to use older libcurl

Older versions of libcurl don't have some of the features we try to
use, in particular curl_multi_setopt(). Check for this in the 'is
libcurl available?' configure test so we disable curl support if the
library is too old.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
