file_name

[Akim Demaille]

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.