Include GnuTLS version as additional version component in Server string

It would be nice if mod_gnutls could add the used (runtime) version of
GnuTLS as an additional version component.

The below patch will add this information while falling back to using
the static version string (plus an indication) if the dynamic version
could not be constructed.

Regards,
BenBE.
drop the -PKIX suffix in the session description

(this tracks changes in gnutls 3.2.9, see GnuTLS commit
426fb86a8a2ff113f9f1d210e12d9a51cc164504)