Fix "-fno-common" build.

Index: endprog.c
--- endprog.c.orig
+++ endprog.c
@@ -293,7 +293,7 @@ modification follow.
 #include <stdlib.h>
 #include "periodic.h"
 
-FILE *Audio;
+extern FILE *Audio;
 
 void endprog (int status) 
 {