%set_build_flags: define LT_SYS_LIBRARY_PATH
    
    This config is to let libtool recognize that our 64bit variant of
    %_libdir is actually on the standard/default library path, so libtool
    doesn't think it has to be hard-wired as RPATH.  This is proper solution
    for libtool RPATH issues described in:
    
      https://docs.fedoraproject.org/en-US/packaging-guidelines/#_removing_rpath
    
    The libtool script/macros (new enough, v2.4.6+) honor this variable when
    it isn't possible to detect the system-wide default library path.  It is
    e.g. able to parse /etc/ld.so.* configuration, but there's no info about
    /usr/lib64 on Fedora.
    
    So to not force everybody to do:
    
       %configure LT_SYS_LIBRARY_PATH=...
    
    ... rather set this system-wide.  This is low-risk change since
    older libtool scripts don't use this variable, and really no other
    tools should.