GCC-10: default to -fno-common by default

GCC-10+ has switched -fno-common on by default.  Previously, this was
causing compilation failures where the .DATA blocks are now used, which
means extern variables need declaring outside of .h files.

This change makes those adjustments.

Fixes #119

configure.ac

@@ -192,7 +192,7 @@ AC_PROG_CPP
 
 # added -Wall for gcc, what about for others?
 if test "x$GCC" = "xyes"; then
-  CFLAGS="-Wall -Wno-implicit-int -fcommon $CFLAGS"
+  CFLAGS="-Wall -Wno-implicit-int -fno-common $CFLAGS"
 fi
 
 # Help finding POSIX functions on some systems

fvwm/virtual.c

@@ -84,6 +84,8 @@ static int last_edge_thickness = 2;
 static void store_desktop_cmd(int, char *);
 static int number_of_desktops(struct monitor *);
 
+struct desktop_cmds	 desktop_cmd_q;
+
 /* ---------------------------- exported variables (globals) --------------- */
 
 /* ---------------------------- local functions ---------------------------- */

fvwm/virtual.h

@@ -28,7 +28,7 @@ struct desktop_cmd {
 	TAILQ_ENTRY(desktop_cmd)	 entry;
 };
 TAILQ_HEAD(desktop_cmds, desktop_cmd);
-struct desktop_cmds	 desktop_cmd_q;
+extern struct desktop_cmds	 desktop_cmd_q;
 
 void apply_desktops_monitor(struct monitor *);
 

libs/FScreen.c

@@ -54,6 +54,12 @@ static void		 monitor_set_flags(struct monitor *);
 static void		 scan_screens(Display *);
 static void		 monitor_check_primary(void);
 
+enum monitor_tracking monitor_mode;
+DesktopsInfo	 *ReferenceDesktops;
+struct screen_infos	 screen_info_q;
+struct monitors		monitor_q;
+int randr_event;
+
 static void GetMouseXY(XEvent *eventp, int *x, int *y)
 {
 	XEvent e;

libs/FScreen.h

@@ -70,8 +70,8 @@ enum monitor_tracking
 	MONITOR_TRACKING_M,
 };
 
-enum monitor_tracking monitor_mode;
-DesktopsInfo	 *ReferenceDesktops;
+extern enum monitor_tracking monitor_mode;
+extern DesktopsInfo	 *ReferenceDesktops;
 
 struct screen_info {
 	const char		*name;
@@ -85,7 +85,7 @@ struct screen_info {
 };
 TAILQ_HEAD(screen_infos, screen_info);
 
-struct screen_infos	 screen_info_q;
+extern struct screen_infos	 screen_info_q;
 
 struct screen_info	*screen_info_new(void);
 struct screen_info	*screen_info_by_name(const char *);
@@ -150,7 +150,7 @@ struct monitor {
 };
 TAILQ_HEAD(monitors, monitor);
 
-struct monitors		monitor_q;
+extern struct monitors		monitor_q;
 
 struct monitor	*monitor_by_name(const char *);
 struct monitor	*monitor_by_xy(int, int);
@@ -166,7 +166,7 @@ void		 monitor_add_new(void);
 
 #define FSCREEN_MANGLE_USPOS_HINTS_MAGIC ((short)-32109)
 
-int randr_event;
+extern int randr_event;
 
 /* Control */
 Bool FScreenIsEnabled(void);

modules/FvwmPager/FvwmPager.c

@@ -62,6 +62,8 @@ PagerStringList *NewPagerStringItem(PagerStringList *last, int desk);
 extern FlocaleFont *FwindowFont;
 extern Pixmap default_pixmap;
 
+struct fpmonitors		 fp_monitor_q;
+
 /*
  *
  * Screen, font, etc info

modules/FvwmPager/FvwmPager.h

@@ -31,7 +31,7 @@ struct fpmonitor {
 };
 TAILQ_HEAD(fpmonitors, fpmonitor);
 
-struct fpmonitors		 fp_monitor_q;
+extern struct fpmonitors	 fp_monitor_q;
 struct fpmonitor		*fpmonitor_by_name(const char *);
 struct fpmonitor		*fpmonitor_by_output(int);
 struct fpmonitor		*fpmonitor_get_current(void);