gcc编译的时候提供了预定义功能，参数是-D, 通过man gcc可以了解更多：
-D name
Predefine name as a macro, with definition 1.
-D name=definition
Predefine name as a macro, with definition definition.
The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive.
In particular, the definition will be truncated by embedded new-line characters.
在
可以通过以下例子解释一下：
int main()
{
#ifdef HELLO
printf("HELLO defined\n");
#else
printf("HELLO not define\n");
#endif
return 0;
}
在该程序中，判断是否已经定义了宏DEBUG,从而输出相应的语句。
如果编译该程序时采用了gcc -DHELLO, 则输出：HELLO defined
（也可以在makefile中直接给变量赋值 DEFS = -DHAVE__PROGNAME=1）