Use #pragma once

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

include/ewmh.h

@@ -7,8 +7,7 @@
  * ewmh.c: Get/set certain EWMH properties easily.
  *
  */
-#ifndef I3_EWMH_C
-#define I3_EWMH_C
+#pragma once
 
 /**
  * Updates _NET_CURRENT_DESKTOP with the current desktop number.
@@ -62,5 +61,3 @@ void ewmh_setup_hints(void);
  *
  */
 void ewmh_update_workarea(void);
-
-#endif