RPM: Allow running 'make rpms' on RHEL 5 machines
    
    Our previous detection for this was flawed, because the %{rhel}
    macro did not exist on the version of RPM shipped with RHEL 5, but
    it worked when building for RHEL 5 through mock. This new patch
    relies on grepping /etc/redhat-release for the version
    information.
    
    https://fedorahosted.org/sssd/ticket/1206