wmmoonclock: fix multiple definitions of some global variables.

A number of variables are declared in a header with no explicit linkage.
This results in there being definitions of them in multiple object
files, which causes a link failure with GCC 10, since this uses
-fno-common by default.

Add `extern` to the header declarations and separate declarations with
no linkage in xutils.c where they are assigned.

Link: https://bugs.debian.org/957950
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>

wmmoonclock/src/xutils.c

@@ -40,6 +40,15 @@
 
 
 
+/*
+ *   Global variable
+ */
+Display		*display;
+Window          Root;
+Window          iconwin, win;
+int             screen;
+int             DisplayDepth;
+
 /*
  *   X11 Variables
  */

wmmoonclock/src/xutils.h

@@ -20,11 +20,11 @@ typedef struct {
 /*
  *   Global variable
  */
-Display		*display;
+extern Display		*display;
-Window          Root;
+extern Window          Root;
-Window          iconwin, win;
+extern Window          iconwin, win;
-int             screen;
+extern int             screen;
-int             DisplayDepth;
+extern int             DisplayDepth;