I think there will be many complaints about the change from filename to file_name in lalr1.cc. In particular, you provide no means to have a version that works both with 2.0 and 2.1. That's nasty.
I do not think this patch is vital, so I would simply reverse it. But if you mean to keep it, there should be a means to choose the name. For instance instead of file_name or filename, you should use b4_position_file_name_name defaulting to file_name, and write in the documentation that %define "b4_position_file_name_name" "filename" guarantees a constant behavior with both versions.
