output: do not generate source files when late errors are caught

Reported by Alexandre Duret-Lutz as "second problem" in:
http://lists.gnu.org/archive/html/bug-bison/2013-09/msg00015.html

* bootstrap.conf: We need the "unlink" module.
* src/files.h, src/files.c (unlink_generated_sources): New.
* src/output.c: Use it.
* tests/output.at: Check the case of late errors.

src/files.c

@@ -398,6 +398,16 @@ output_file_name_check (char **file_name, bool source)
     }
 }
 
+void
+unlink_generated_sources (void)
+{
+  int i;
+  for (i = 0; i < generated_files_size; i++)
+    if (generated_files[i].is_source)
+      /* Ignore errors.  The file might not even exist.  */
+      unlink (generated_files[i].name);
+}
+
 void
 output_file_names_free (void)
 {

src/files.h

@@ -70,6 +70,9 @@ void output_file_names_free (void);
  */
 void output_file_name_check (char **file_name, bool source);
 
+/** Remove all the generated source files. */
+void unlink_generated_sources (void);
+
 FILE *xfopen (const char *name, char const *mode);
 void xfclose (FILE *ptr);
 FILE *xfdopen (int fd, char const *mode);

src/output.c

@@ -704,6 +704,11 @@ output (void)
   /* Process the selected skeleton file.  */
   output_skeleton ();
 
+  /* If late errors were generated, destroy the generated source
+     files. */
+  if (complaint_status)
+    unlink_generated_sources ();
+
   obstack_free (&format_obstack, NULL);
 }