Made `+=' more robust. Fixes pluseq6.test and pluseq7.test. * automake.in (define_standard_variables): New sub. (read_main_am_file): Use two passes to scan standard variables. (read_am_file): Don't special-case configure variables with `+='. (initialize_per_input): Initialize %var_was_plus_eq. (read_am_file): Set var_was_plus_eq element correctly. (define_variable): Give error if variable assigned with `+=' is internally defined. (initialize_per_input): Initialize %am_var_defs. (file_contents_with_transform): Set element in %am_var_defs. (read_am_file): Use %am_var_defs.