Fix building ngIRCd with OpenSSL 1.1

Fix building ngIRCd with OpenSSL 1.1

At the moment, ngIRCd fails to build against OpenSSL 1.1 since the
configure check probes for the SSL_library_init symbol which was
removed, but probing for a different function availabe in both versions
solves that problem: SSL_new().

And as SSL_library_init is no longer needed, the patch boils down to
probing SSL_new to assert libssl is available, and disabling the
SSL_library_init invokation from OpenSSL 1.1 on, see also another
application[1] (NSCA-ng) that did pretty much the same.

Patch was compile-tested on both Debian jessie (OpenSSL 1.0.2) and
stretch (OpenSSL 1.1).

[1] <https://www.nsca-ng.org/cgi-bin/repository/nsca-ng/commit/?id=8afc22031ff174f02caad4afc83fa5dff2c29f8a>

(Patch by Christoph, commit message cherry-picked from the email thread
on the mailing list by Alex. Thanks!)