error.c has an unnecessary dependency on "program_name"

[Bruce Korb]

It is fairly easy to fix, just make the definition weak
and check for a NULL value before using it.  OK to push?

index ed9dba0..65f2d9d 100644
--- a/lib/error.c
+++ b/lib/error.c
@@ -106,7 +106,7 @@ char *strerror_r ();

 /* The calling program should define program_name and set it to the
    name of the executing program.  */
-extern char *program_name;
+char *program_name;

 # if HAVE_STRERROR_R || defined strerror_r
 #  define __strerror_r strerror_r
@@ -300,10 +300,11 @@ error (int status, int errnum, const char *message, ...)
     (*error_print_progname) ();
   else
     {
+      char const *p = program_name ? program_name : "unknown";
 #if _LIBC
-      __fxprintf (NULL, "%s: ", program_name);
+      __fxprintf (NULL, "%s: ", p);
 #else
-      fprintf (stderr, "%s: ", program_name);
+      fprintf (stderr, "%s: ", p);
 #endif
     }

@@ -359,10 +360,11 @@ error_at_line (int status, int errnum, const char 
*file_name,
     (*error_print_progname) ();
   else
     {
+      char const *p = program_name ? program_name : "unknown";
 #if _LIBC
-      __fxprintf (NULL, "%s:", program_name);
+      __fxprintf (NULL, "%s: ", p);
 #else
-      fprintf (stderr, "%s:", program_name);
+      fprintf (stderr, "%s: ", p);
 #endif
     }