add missing declaration of old_x11

src/frontend/plotting/grid.c

@@ -36,6 +36,10 @@ extern bool old_x11;
 extern int X11_GetLenStr(GRAPH *gr, char* instring);
 #endif
 
+#ifndef X_DISPLAY_MISSING
+extern bool old_x11;
+#endif
+
 #define RAD_TO_DEG      (180.0 / M_PI)
 #define RELPOSXUNIT 0.6 /* old position of the UNIT label */
 

src/frontend/plotting/x11.c

@@ -60,6 +60,8 @@ Author: 1988 Jeffrey M. Hsu
 
 #define NUMCOLORS 20
 
+bool old_x11 = FALSE; /* if FALSE, use xft, if TRUE use old X11 text output */
+
 typedef struct x11info {
     Window window;
     int isopen;