Changes to compile on CentOS, Fedora and Redhat

OpenSSL on CentOS, Fedora and RedHat does not have an implementation of NID_secp256k1. I suspect some other platforms may be missing it too. On these platforms namecoin throws an exception and exists upon start since calls to EC_GROUP_new_by_curve_name(NID_secp256k1) return NULL. This patch provides an alternative implementation that is kicked in unless the HAVE_NID_secp256k1 macro is defined.

This is based on similar patch in the bitcoin https://bitcointalk.org/index.php?topic=364877.0