经典的互斥锁线程例子，使用 pthread 的互斥锁机制，使得程序运行的结果正确,使用 pthread 线程库创建两个线程，两个线程都要访问共享变量counter，按照源程序的逻辑，两个线程各循环 5 次，每次循环对 counter 变量加 1，最后打印的结果是 10。-The classic example of mutex thread using pthread mutex lock mechanism, so that the results of the program is running correctly, use pthread threading library to create two threads, two threads must access a shared variable counter, according to the source logic, two threads each cycle five times, each time adding a circulation of counter variables, the results of the final print is 10.