rpm: define weak_deps for el8

rpm: define weak_deps for el8

RHEL/CentOS 8 comes with rpm 4.14, see
https://centos.pkgs.org/8/centos-baseos-x86_64/rpm-4.14.2-25.el8.x86_64.rpm.html
and
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/packaging_and_distributing_software/new-features-in-rhel-8_packaging-and-distributing-software

and since "Recommends" was introduced by rpm 4.12, see
https://fedoraproject.org/wiki/Changes/RPM-4.12 .
so we are able to use "Recommends" in el8 as well.

Signed-off-by: Kefu Chai <kchai@redhat.com>