Fix #73: resolve strptime() implicit declaration

This silences the compiler warning:
```
sunmenu.c: In function ‘readSOHOImage’:
sunmenu.c:926:3: warning: implicit declaration of function ‘strptime’; did you mean ‘strftime’? [-Wimplicit-function-declaration]
  926 |   strptime(buf+20, "%d %b %Y %H:%M:%S %Z", &tm);
```

GUI/xephem/sunmenu.c

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