fix build with GCC 10 and upper

GCC switched default from -fcommon to -fno-common,
leading to the following compilation errors:

multiple definition of `argv0'
multiple definition of `display'
multiple definition of `scr'
multiple definition of `font'
multiple definition of `cnorm'
multiple definition of `csel'
multiple definition of `debugflag'
multiple definition of `debugfile'
multiple definition of `cursor'
multiple definition of `framehandler'
multiple definition of `NM'

cmd/menu/dat.h

@@ -93,9 +93,9 @@ EXTERN struct {
 	int	maxwidth;
 } match;
 
-Font*		font;
-CTuple		cnorm;
-CTuple		csel;
+extern Font*		font;
+extern CTuple		cnorm;
+extern CTuple		csel;
 
 EXTERN Item	hist;
 EXTERN Item*	histsel;

cmd/menu/main.c

@@ -12,6 +12,13 @@
 #include "fns.h"
 #define link _link
 
+char *argv0;
+Display *display;
+Screen scr;
+Font* font;
+CTuple cnorm;
+CTuple csel;
+
 static const char version[] = "wimenu-"VERSION", "COPYRIGHT"\n";
 static Biobuf*	cmplbuf;
 static Biobuf*	inbuf;

cmd/strut/dat.h

@@ -12,6 +12,6 @@
 
 enum { DAuto, DHorizontal, DVertical };
 
-EXTERN Handlers	handlers;
+extern Handlers	handlers;
 EXTERN int	direction;
 

cmd/strut/main.c

@@ -9,6 +9,10 @@
 #include <time.h>
 #include "fns.h"
 
+char *argv0;
+Display *display;
+Screen scr;
+
 static Window*	testwin;
 static ulong	testtime[2];
 

cmd/tray/main.c

@@ -11,6 +11,10 @@
 #include <sys/signal.h>
 #include "fns.h"
 
+char *argv0;
+Display *display;
+Screen scr;
+
 static const char version[] = "witray-"VERSION", "COPYRIGHT"\n";
 
 static int	exitsignal;

cmd/wmii/dat.h

@@ -42,7 +42,7 @@ enum {
 	CurNone,
 	CurLast,
 };
-Cursor	cursor[CurLast];
+extern Cursor	cursor[CurLast];
 
 enum IncMode {
 	IIgnore,
@@ -355,11 +355,11 @@ EXTERN Key*	key;
 EXTERN View*	selview;
 EXTERN View*	view;
 
-EXTERN Handlers	framehandler;
+extern Handlers	framehandler;
 
 /* IXP */
 EXTERN IxpServer srv;
-EXTERN Ixp9Srv	p9srv;
+extern Ixp9Srv	p9srv;
 
 /* X11 */
 EXTERN uint	numlock_mask;

cmd/wmii/debug.h

@@ -20,5 +20,5 @@ void	dwrite(int, void*, int, bool);
 bool	setdebug(int);
 void	vdebug(int, const char*, va_list);
 
-long	debugflag;
-long	debugfile;
+extern long	debugflag;
+extern long	debugfile;

cmd/wmii/main.c

@@ -14,6 +14,13 @@
 #include <sys/stat.h>
 #include "fns.h"
 
+char *argv0;
+Display *display;
+Screen scr;
+Cursor cursor[CurLast];
+long debugflag;
+long debugfile;
+
 static const char
 	version[] = "wmii-"VERSION", "COPYRIGHT"\n";
 

cmd/wmiir.c

@@ -20,6 +20,8 @@
 #include <bio.h>
 #include <fmt.h>
 
+char *argv0;
+
 static IxpClient* client;
 static Biobuf*	outbuf;
 static bool	binary;

cmd/x11/wikeyname.c

@@ -7,6 +7,10 @@
 #include <locale.h>
 #include <unistd.h>
 
+char *argv0;
+Display *display;
+Screen scr;
+
 static const char version[] = "wikeyname-"VERSION", "COPYRIGHT"\n";
 
 static Handlers handlers;

cmd/x11/wmii9menu.c

@@ -48,6 +48,10 @@
 #include <stuff/util.h>
 #include <stuff/x.h>
 
+char *argv0;
+Display *display;
+Screen scr;
+
 char version[] = "wmii9menu-"VERSION" "COPYRIGHT", ©1994 David Hogan, Arnold Robbins";
 
 static Window*	menuwin;

include/stuff/x11.h

@@ -214,8 +214,8 @@ struct Screen {
 # pragma varargck	type	"W"	Window*
 #endif
 
-Display *display;
-Screen scr;
+extern Display *display;
+extern Screen scr;
 
 extern char*		modkey_names[];
 extern struct Map	windowmap;

lib/libstuff/map.c

@@ -13,7 +13,7 @@ struct MapEnt {
 	MapEnt*		next;
 };
 
-MapEnt *NM;
+static MapEnt *NM;
 
 /* By Dan Bernstein. Public domain. */
 static ulong