Add support for building with shared OpenSSL lacking SSLv3.

Some distributions are disabling SSLv3 due to the POODLE attack.  For example, the OpenSSL in Debian experimental has removed support for SSLv3_method and friends.  If OpenSSL has SSLv3 disabled, throw an exception, just like when SSLv2 is disabled.
Does SSLv23_method() still exist when OPENSSL_NO_SSL3 is defined?