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

threads-sema/rwlock.c

@@ -1,12 +1,17 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <pthread.h>
-#include <semaphore.h>
 #include <unistd.h>
 
 #include "common.h"
 #include "common_threads.h"
 
+#ifdef linux
+#include <semaphore.h>
+#elif __APPLE__
+#include "zemaphore.h"
+#endif
+
 typedef struct _rwlock_t {
     sem_t writelock;
     sem_t lock;