Do not use deprecated openssl function

RSA_generate_key is marked as deprecated and as been remplaced by
RSA_generate_key_ex.
Some distributors, like Android, do not support deprecated openssl.

This change update requirement to use openssl >= 0.9.8. This version was
released in 2005, Debian ship it since Etch (2007) and Redhat since
RHEL5 (2007)