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)
 {