Minimal version seems to be the one that was in Fedora 10: https://fedoraproject.org/wiki/Packaging:Guidelines#BuildRoot_tag
No, that's not a rpm version but a redhat-rpm-config thing, and newspec's goal is to produce specfiles that work/build properly also when it's not installed.
https://bugzilla.redhat.com/show_bug.cgi?id=807857
This is sad, because IIRC people are removing this line from SPECs and it is also causing trouble: https://bugzilla.redhat.com/show_bug.cgi?id=1139501
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