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

[Paul Eggert]

Eli Zaretskii wrote:
> the GnuTLS manual doesn't say for each function in what version
> it was introduced

The GnuTLS folks are pretty good about documenting API changes, it's just not in 
the main manual. For example:

http://www.gnutls.org/abi-tracker/timeline/gnutls/

I used that list, along with a copy of the GnuTLS development history, to come 
up with the version numbers in the proposed patch.  The patch should not change 
Emacs functionality at all: all it should do is simplify configuration (and make 
'configure' run a bit faster :-).