Use #pragma once

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

include/i3.h

@@ -7,8 +7,7 @@
  * i3.h: global variables that are used all over i3.
  *
  */
-#ifndef I3_I3_H
-#define I3_I3_H
+#pragma once
 
 #include <sys/time.h>
 #include <sys/resource.h>
@@ -62,5 +61,3 @@ extern bool xcursor_supported, xkb_supported;
 extern xcb_window_t root;
 extern struct ev_loop *main_loop;
 extern bool only_check_config;
-
-#endif