Fix for -fno-common

Index: src/parse_config.h
--- src/parse_config.h.orig
+++ src/parse_config.h
@@ -85,7 +85,7 @@ typedef struct colours
 colours;
 #endif /* NO_COLOR_CURSES */
 
-int use_manual;
+extern int use_manual;
 
 int parse_config (void);
 int parse_line (char *line);