Clean up gnutls warnings.

Clean up gnutls warnings.

Most of the warning were caused by deprecated types:
  gnutls_certificate_credentials
  gnutls_certificate_status
  gnutls_datum
  gnutls_digest_algorithm
  gnutls_session
  gnutls_transport_ptr
  gnutls_x509_crt
Even though I believe the replacements have been around for a while, the
patch adds autoconf checks and fallback typedefs.

One warning was caused by casting an int to a pointer for the second
parameter to gnutls_transport_set_ptr().  Recent gnutls has a
replacement gnutls_transport_set_int() macro, but this macro simply
(eventually) casts the parameter using "(gnutls_transport_ptr_t)(long)".
So this patch just does the same.