Made `+=' more robust. Fixes pluseq6.test and pluseq7.test.

	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.