Phase out buildroot tag (draft)

BuildRoot tag
The BuildRoot value MUST be below  and MUST contain at least ,   and. It may invoke  since this is guaranteed to exist on every system. From there, packagers are expected to use a sane BuildRoot.

The recommended values for the BuildRoot tag are (in descending order of preference) : %(mktemp -ud %{_tmppath}/%{name}-%{version}-%{release}-XXXXXX) %{_tmppath}/%{name}-%{version}-%{release}-root-%(%{__id_u} -n) %{_tmppath}/%{name}-%{version}-%{release}-root

At one point, the second was a mandatory value, but it is now left to the packager to decide. If unsure, simply pick the first.