crypto: introduce build system for gnutls crypto backend

This introduces the build logic needed to decide whether we can
use gnutls as a crypto driver backend. The actual implementations
will be introduced in following patches. We only wish to use
gnutls if it has version 3.6.14 or newer, because that is what
finally brings HW accelerated AES-XTS mode for x86_64.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>