Build: let "make [s]rpm" use pre-/post-release logic from spec proper

This effectively reverts c071e92 that wasn't necessarily correct
as one should be still able to pass "--define '_with_FOO --with-FOO'"
to rpmbuild as an equivalent substitute to "--with FOO", and elaborates
on this one-authoritative-point-of-reasoning-about-the-release idea
so as to overcome the previously observed obstacles.