Make X25519 support depend on the Libgcrypt version
* src/ntbtls-int.h (SUPPORT_X25519): New. * src/ecdh.c (_ntbtls_ecdh_read_params): Include X25519 only with modern libgcrypt versions. * src/protocol-cli.c (write_supported_elliptic_curves_ext): Announce X25519 only if we support it.
We better stay conservative and don't support the new algorithms with
the 1.8 LTS version of Libgcrypt.