Merge pull request #36 from saifeiLee/t/fix-6-06

fix: enum directive_type UNDEF and DEFINE should be different

Author: ohkimur

chapter_6/exercise_6_06/define.c

@@ -17,7 +17,7 @@ enum directive_type
 {
   NONE = 0,
   DEFINE = 1,
-  UNDEF = 1
+  UNDEF = 2
 };
 
 struct list_node