./configure.ac: detect SSL in libcurl using curl-config

./configure.ac: detect SSL in libcurl using curl-config

The API of libcurl does not mention Curl_ssl_init() and when curl is
built with -flto, the Curl_ssl_init symbol is not exported.

https://curl.haxx.se/libcurl/using/ suggests calling

  curl-config --feature | grep SSL

to see, if the installed curl has SSL support.  Another approach
would be calling curl_version_info and checking the returned struct.

This patch removes the check for the Curl_ssl_init exported symbol
from libcurl and uses curl-config to detect SSL support in libcurl.

Signed-off-by: Дилян Палаузов <git-dpa@aegee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>