fix warning about unused label on non win32 build

- moved platform dependend code into scope its respective #ifdef
- killed one useless 'goto' with great pleasure
- introduced duplicate code to preserve behavior (fallthrough case)

common/JackMessageBuffer.cpp

@@ -189,13 +189,12 @@ int JackMessageBuffer::SetInitCallback(JackThreadInitCallback callback, void *ar
             fGuard.Signal();
             JackSleep(1000);
         }
-        if (count == 1000) goto error;
+        if (count == 1000)
+            jack_error("JackMessageBuffer::SetInitCallback : callback cannot be executed");
+            return -1;
     #endif
-    
         return 0;
     }
-    
-error:
     jack_error("JackMessageBuffer::SetInitCallback : callback cannot be executed");
     return -1;
 }