lftp should be build w/ gnutls support

lftp is currently built w/o gnutls support therefore doesn't support secure ftp connections since it can't validate certificates. in this age of internet all services rely on secure connections, why lftp should be also build w/ gnutls support.

current version of lftp:

~ # cat /etc/os-release 
NAME="Alpine Linux"
ID=alpine
VERSION_ID=3.12_alpha20191219
PRETTY_NAME="Alpine Linux edge"
HOME_URL="https://alpinelinux.org/"
BUG_REPORT_URL="https://bugs.alpinelinux.org/"

~ # cat /etc/apk/repositories 
http://dl-cdn.alpinelinux.org/alpine/edge/main
http://dl-cdn.alpinelinux.org/alpine/edge/community

~ # lftp --version
LFTP | Version 4.9.0 | Copyright (c) 1996-2019 Alexander V. Lukyanov

LFTP is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with LFTP.  If not, see <http://www.gnu.org/licenses/>.

Send bug reports and questions to the mailing list <lftp@uniyar.ac.ru>.

Libraries used: Readline 8.0