configure.ac: detect ssl need with libcurl

configure.ac: detect ssl need with libcurl

When libcurl has been statically compiled with openssl support they both
need to be linked in everytime libcurl is used.

During configuration this can be detected by looking for Curl_ssl_init
function symbol in libcurl, which will only be present if libcurl has been
compiled statically built with openssl.

configure.ac checks for Curl_ssl_init function in libcurl and if such function
exists; it sets NEEDS_SSL_WITH_CURL that is used by the Makefile to include
-lssl alongside with -lcurl.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>