parse '+=' after a requires the same way as '=' for now.
parse '+=' after a requires the same way as '=' for now.

Most of times += is actually equivalent to = as apparently it's very often misused.

improves slightly the error message to actually be comprehensible. just.

fixes #20.

ocaml-obuild/obuild,ocaml-obuild/obuild,z4p4n/obuild,UnixJunkie/obuild,z4p4n/obuild,UnixJunkie/obuild