made it so that zemaphore is used on mac, semaphore on linux, and other clean up

threads-sema/producer_consumer_works.c

@@ -3,11 +3,16 @@
 #include <assert.h>
 #include <pthread.h>
 #include <stdlib.h>
-#include <semaphore.h>
 
 #include "common.h"
 #include "common_threads.h"
 
+#ifdef linux
+#include <semaphore.h>
+#elif __APPLE__
+#include "zemaphore.h"
+#endif
+
 int max;
 int loops;
 int *buffer;