How to compare strings in C conditional preprocessor-directives

想使用 #define 寫code 在 preprocessor 裡，會顯示錯誤：

Invalid token at start of preprocessor expression

解法：

https://stackoverflow.com/questions/2335888/how-to-compare-strings-in-c-conditional-preprocessor-directives

解法 1，換成流水號：

#define USER_JACK 1
#define USER_QUEEN 2

#define USER USER_JACK 

#if USER == USER_JACK
#define USER_VS USER_QUEEN
#elif USER == USER_QUEEN
#define USER_VS USER_JACK
#endif

Or you could refactor the code a little and use C code instead.

解法 2：