Clarify the use of path macros with respect to build dependencies
    
    Packages should not use %{_prefix} and friends to refer to absolute
    paths for files provided by build dependencies, since this breaks
    rebuilding a package with a different prefix than the system.
    
    See https://lists.fedorahosted.org/archives/list/packaging@lists.fedoraproject.org/thread/BILZSHLQ7Y2GFIP3AMYNB6FFOFTLQNGM/