sunmenu: Define _GNU_SOURCE, so that <time.h> declares strptime

The function isan X/Open extension that is not available by default in
the glibc headers.  Currently, this works because compilers support
implicit function declarations, but future versions are likely to
disable by them default, to improve type safety.

GUI/xephem/sunmenu.c

@@ -1,6 +1,9 @@
 /* code to manage the sun display 
  */
 
+/* For the declaration of strptime in <time.h>. */
+#define _GNU_SOURCE
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>