Always put auxiliary code files in the same dir as other output files.

* src/files.c (compute_file_name_parts): When the user specifies
--output but not --file-prefix, extract the directory prefix from the
file prefix not from the grammar file name.  This affects the location
of files like location.hh generated by the C++ skeleton.  The includes
in the other output files require this fix.
* tests/output.at (AT_CHECK_OUTPUT): Automatically create directories
for expected output files.
(Output files): Add a test for the above.

src/files.c

@@ -267,7 +267,9 @@ compute_file_name_parts (void)
       if (spec_file_prefix)
 	{
 	  /* If --file-prefix=foo was specified, ALL_BUT_TAB_EXT = `foo'.  */
-	  dir_prefix = xstrndup (grammar_file, base - grammar_file);
+	  dir_prefix =
+            xstrndup (spec_file_prefix,
+                      last_component (spec_file_prefix) - spec_file_prefix);
 	  all_but_tab_ext = xstrdup (spec_file_prefix);
 	}
       else if (yacc_flag)