Allow 'make rpm' without the need for env-setup

As it used to be before, 'make rpm' from a cloned repository should work without the need to run hacking/env-setup. It's an easy fix, in line with what we already did for the man-pages.
Merge pull request #2299 from dagwieers/rpm-docsite-fix

Allow 'make rpm' without the need for env-setup