Turns out cmake's cpack rpm module is using --buildroot to preinstall
content to a known location and then point a dummy rpmbuild spec to
that location using --buildroot.
This is not how rpmbuild wants to be used but we can't very well break cpack
(and who knows what else may be using it) just like that, commit
9d35c8df49 went a step too far with the
buildroot elimination.
Do a partial revert of 9d35c8df49 to
honor buildRoot argument to rpmSpecParse() again and bring back
--buildroot cli switch to set it, just enough to cover this usage. Also
add a test-case to mimick the "install from the outside" use that cpack does.
996b3ac32d