gnurl: fall-back on default system trust store

gnurl: fall-back on default system trust store

If no explicit CA file is given, gnurl fails to setup HTTPS connections
as it doesn't looks for certificates in /etc/ssl/certs/ in any way.
Fix that by utilizing GnuTLS' gnutls_certificate_set_x509_system_trust
as a fall-back if neither CA file, CA path nor SRP is declared.

Reported upstream: https://github.com/bagder/curl/issues/330
Fix suggested upstream: https://github.com/bagder/curl/pull/331

Signed-off-by: Daniel Golle <redacted>