I don't think we should do that. IMO we must not modify the source tree in any way when building in a separate dir.
It depends which files we're talking about I think that *~ (and any editor backup files), *rej* *orig* (patch leftovers), .#* (cvs update leftovers) should be removed from the source tree perhaps we need to seperate the semantics from clean and distclean. but at least, from a distclean point of view, I do think it would be great to remove the hereabove mentionned files from the source tree A+