Fix the build with -fno-common

Index: wmgeneral.h
--- wmgeneral.h.orig
+++ wmgeneral.h
@@ -41,10 +41,10 @@ typedef struct {
  /* Global variable */
 /*******************/
 
-Display *display;
-Window Root, iconwin, win;
-XpmIcon wmgen;
-XpmIcon wmfont;
+extern Display *display;
+extern Window Root, iconwin, win;
+extern XpmIcon wmgen;
+extern XpmIcon wmfont;
 
 /***********************/
  /* Function Prototypes */
