bugfix: gnutls version 2.9.10 required, but 1.4.0 requested

The reason is that we need the gnutls_certificate_set_verify_function
in order to do decent certificate verfication. This function did not
appear before 2.9.10. With the current setting, the library build on
systems with too-old GnuTLS, but executing programs using it lead
to load-time errors and abort