[C/C++] C Macro token concatenation

取自wiki
http://en.wikipedia.org/wiki/C_preprocessor

Code Example:
#define MYCASE(item,id) 
　case id: 
　　item##_##id = id;
　break;

Example Result: 
switch(x) {
　MYCASE(widget,23);
}

------------------------------------------------- ##可以連接程式碼token