Fix build when OpenSSL is built without SSLv3

Fix build when OpenSSL is built without SSLv3

If OpenSSL is built without SSLv3 support (configure --no-ssl3), this will fail to build. Patch fixes that. Please see https://github.com/pcbsd/freebsd-ports/blob/master/irc/znc/files/patch-src_Csocket.cpp as well. This is intended to be applied to the FreeBSD ports tree as well.

As a side note: You may wish to refactor the code to only use SSLv23_ methods and set SSL_OP_NO_* using SSL_CTX_set_options. This seems to be the canonical way to be able to negotiate any SSL/TLS version. In addition, OpenSSL 1.1 will be marking SSLv23_ methods as deprecated and replace them with TLS_ methods.