Use #pragma once

#pragma once is safer and simpler. According to Wikipedia it's supported by all major compilers.

include/i3/ipc.h

@@ -8,8 +8,7 @@
  * for the IPC interface to i3 (see docs/ipc for more information).
  *
  */
-#ifndef I3_I3_IPC_H
-#define I3_I3_IPC_H
+#pragma once
 
 #include <stdint.h>
 
@@ -101,5 +100,3 @@ typedef struct i3_ipc_header {
 
 /** Bar config update will be triggered to update the bar config */
 #define I3_IPC_EVENT_BARCONFIG_UPDATE           (I3_IPC_EVENT_MASK | 4)
-
-#endif