Fix build with "-fno-common".

Index: tsiag/window.c
--- tsiag/window.c.orig
+++ tsiag/window.c
@@ -98,7 +98,7 @@ MwFmt fmt1 = {"Helvetica", 120, 0, 0, 0, 0, "black", "
 
 int columns;    /* number of cells per line */
 int width;      /* default cell width */
-int left_margin;
+extern int left_margin;
 /*int right_margin = 5;*/
 
 float zoom = 1.0;	/* unused */
