The sntrup761 code sourced from supercop uses variable length
arrays.  Although widely supported, they are not part of the ANSI
C89 spec so if the compiler does not support VLAs, disable the
sntrup761x25519-sha512@openssh.com KEX method by replacing the kex
functions with no-op ones similar to what we do in kexecdh.c.

This should allow OpenSSH to build with a plain C89 compiler again.
Spotted by tim@, ok djm@.