bug#27708: [PROPOSED] Simplify configuration of HAVE_GNUTLS3 etc.

[Eli Zaretskii]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 15 Jul 2017 09:14:05 -0700
> Cc: Paul Eggert <eggert@cs.ucla.edu>
> 
> diff --git a/src/gnutls.h b/src/gnutls.h
> index 3ec86a8..19c1686 100644
> --- a/src/gnutls.h
> +++ b/src/gnutls.h
> @@ -23,8 +23,16 @@ along with GNU Emacs.  If not, see 
> <http://www.gnu.org/licenses/>.  */
>  #include <gnutls/gnutls.h>
>  #include <gnutls/x509.h>
>  
> -#ifdef HAVE_GNUTLS3
> -#include <gnutls/crypto.h>
> +#if 0x030000 <= GNUTLS_VERSION_NUMBER
> +# define HAVE_GNUTLS3
> +# include <gnutls/crypto.h>
> +#endif
> +
> +#if 0x030400 <= GNUTLS_VERSION_NUMBER
> +# define HAVE_GNUTLS3_AEAD
> +# define HAVE_GNUTLS3_CIPHER
> +# define HAVE_GNUTLS3_DIGEST
> +# define HAVE_GNUTLS3_HMAC
>  #endif

If we want to support the new APIs only starting with GnuTLS 3.4.0,
then this is a step in the right direction.  But is that the intent?
Most of the functions we need are available in much older versions,
and others since 3.2.0.  Only a few appeared in 3.4.0.  So it might
also make sense to make our code more fine-grained, not less, if we
want to make as many of these APIs available on as many platforms as
possible.

But I'm not sure what was Ted's intent, and what we want as a project.