sunmenu.c:920:3: warning: implicit declaration of function ‘strptime’

--- a/GUI/xephem/sunmenu.c
+++ b/GUI/xephem/sunmenu.c
@@ -1,6 +1,7 @@
 /* code to manage the sun display 
  */
 
+#define _XOPEN_SOURCE /* See feature_test_macros(7) */
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>