Build: rpm: require yum-builddep to do "make rpm-dep"

rpm-dep without yum-builddep required grep'ing the spec file for BuildRequires.
That's been broken since 1.1.16 via 62859240, which used pkgconfig() in a
BuildRequires -- and further commits since then have also used pkgconfig() or a
global.

There's no point in trying to fix it since it was always hacky, and no one's
complained since it's been broken.