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

threads-sema/zemaphore.c

@@ -5,34 +5,7 @@
 
 #include "common.h"
 #include "common_threads.h"
-
-typedef struct __Zem_t {
-    int value;
-    pthread_cond_t  cond;
-    pthread_mutex_t lock;
-} Zem_t;
-
-void Zem_init(Zem_t *z, int value) {
-    z->value = value;
-    Cond_init(&z->cond);
-    Mutex_init(&z->lock);
-}
-
-void Zem_wait(Zem_t *z) {
-    Mutex_lock(&z->lock);
-    while (z->value <= 0)
-	Cond_wait(&z->cond, &z->lock);
-    z->value--;
-    Mutex_unlock(&z->lock);
-}
-
-void Zem_post(Zem_t *z) {
-    Mutex_lock(&z->lock);
-    z->value++;
-    Cond_signal(&z->cond);
-    Mutex_unlock(&z->lock);
-}
-
+#include "zemaphore.h"
 
 Zem_t s;