Made the rpm-related pkg-config configuration more robust.
Made the rpm-related pkg-config configuration more robust.

The same target of the check (`rpm`) screwed up other checks.
Checks for different `librpm` versions have `rpm4`, rpm5`, ... targets
that are then transferred to the `rpm_...` env variables.

Fixes: #453

OpenSCAP/openscap,jan-cerny/openscap,OpenSCAP/openscap,redhatrises/openscap,jan-cerny/openscap,mpreisler/openscap,OpenSCAP/openscap,jan-cerny/openscap,OpenSCAP/openscap,mpreisler/openscap,mpreisler/openscap,OpenSCAP/openscap,redhatrises/openscap,mpreisler/openscap,redhatrises/openscap,jan-cerny/openscap,jan-cerny/openscap,redhatrises/openscap,mpreisler/openscap,mpreisler/openscap,redhatrises/openscap,OpenSCAP/openscap,jan-cerny/openscap,redhatrises/openscap