[gnutls-help] hashing in older versions

[Jeremy Harris]

I want a sha256/sha1 hash, which can be built
incrementally (we don't want to slurp an entire
mail body, which might be tens of MB) so can't use gnutls_fingerprint().
 gnutls_hash() &c were only
introduced as supported routines in 2.10.0 and
RHEL6.5 - age systems are back with 2.8.5,
but the Exim project still wishes to support them.

What should I use?

-- 
Thanks,
  Jeremy