bcrypt: Ensure we have gnutls_hash() available.

[Huw Davies]

On 18 Jan 2016, at 12:00, Huw Davies wrote:
> On 18 Jan 2016, at 11:56, Sebastian Lackner wrote:
>> 
>> Wouldn't it be useful to warn a bit more explicit at compile-time? Otherwise packagers will
>> have a hard time to find out if its supported by their build.
> 
> I figured that since this only affects fairly old versions of gnutls, that the extra
> bloat wouldn't be worth it.

Actually, I guess it's not too bad, I'll resend with a warning.

Thanks,
Huw.