https://bugs.gentoo.org/706256

Fix build failure on gcc-10/-fno-common.

--- a/src/shar-opts.h
+++ b/src/shar-opts.h
@@ -352,7 +352,7 @@ extern "C" {
  *  global exported definitions
  */
 #include "local.h"
-char const * const program_name;
+extern char const * const program_name;
 
 extern bool initialization_done;
 extern int optidx;
--- a/src/unshar-opts.h
+++ b/src/unshar-opts.h
@@ -192,7 +192,7 @@ extern "C" {
 extern size_t       separator_str_len;
 
 #include "local.h"
-char const * const program_name;
+extern char const * const program_name;
 
 
 /* * * * * *
--- a/src/uudecode-opts.h
+++ b/src/uudecode-opts.h
@@ -170,7 +170,7 @@ extern "C" {
  *  global exported definitions
  */
 #include "local.h"
-char const * const program_name;
+extern char const * const program_name;
 
 
 /* * * * * *
--- a/src/uuencode-opts.h
+++ b/src/uuencode-opts.h
@@ -166,7 +166,7 @@ extern "C" {
  *  global exported definitions
  */
 #include "local.h"
-char const * const program_name;
+extern char const * const program_name;
 
 
 /* * * * * *