Hi and welcome to LQ,
i am on linux RH6 (it's old !)
If you mean Readhat 6.0, it's not just old. It's ancient!!! and I don't think you'll be able to compile recent software from sources.
But I think you mean Redhat Enterprise Linux 6, which is the latest release. In this case you need to install the openssl-dev-1.0.0d package