The elm specfile in piglet (also the same in rawhide) uses a Buildroot hard coded to /var/tmp/%{name}-root. It makes more sense to use %{_tmppath}/%{name}-root just so you can have the build root where you want/need it due to space considerations.
It ain't just elm that codes a build root that way. Anyways, the Right Thing To Do is to put %_buildroot %{_tmppath}/%{name}-root as part of the build system configuration. That will take precedence over everything but the command line, and just *delete* BuildRoot: from spec files entirely. Marking as WONTFIX in the sense CANTFIX because of legacy packaging.