C中，枚举类型就是int。
如下摘自rationale for the C99 standard
6.4.4.3 Enumeration constants
Whereas an enumeration variable may have any integer type that correctly represents all its
values when widened to int, an enumeration constant is only usable as the value of an
expression. Hence its type is simply int.
枚举变量可以有任意整数类型，当枚举扩展到int时，就可以表示所有的值。所以，枚举的类型就是int。
