fix redefine of #weak_define

fix redefine of #weak_define

This patch fixes 2 bugs in do_handle_define()

1.	if (sym->weak)
		goto replace_it;

	This is wrong, we should allocate a new symbol if
	sym->scope != file_scope, example:

	inc.h:
		#weak_define FOO default

	redefine.c:
		#define FOO redefine
		FOO

	default.c:
		FOO

	$ ./test-lexing -include inc.h redefine.c default.c
	redefine
	Segmentation fault

2.	if (token_list_different(...)) {
		...
	}
	return 1;

	This is wrong too:

		#weak_define	FOO	1
		#define		FOO	1	// has no effect
		#define		FOO	2	// redefined without warning

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>