Fix the build with -fno-common

Index: wmgeneral/wmgeneral.h
--- wmgeneral/wmgeneral.h.orig
+++ wmgeneral/wmgeneral.h
@@ -36,8 +36,8 @@ typedef struct {
  /* Global variable */
 /*******************/
 
-Display		*display;
-Window          Root, iconwin, win;
+extern Display		*display;
+extern Window          Root, iconwin, win;
 
 
   /***********************/
