(OUTPUT_EXT, TAB_EXT): Define only if not defined

already.  Let config.h define any nonstandard values.

src/system.h

@@ -1,6 +1,7 @@
 /* System-dependent definitions for Bison.
 
-   Copyright (C) 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
+   Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005 Free Software
+   Foundation, Inc.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -170,21 +171,13 @@ do {								\
 | Extensions to use for the output files.  |
 `-----------------------------------------*/
 
-#ifdef VMS
-  /* VMS. */
-# define TAB_EXT	"_tab"
-# define OUTPUT_EXT	".output"
-#else /* ! VMS */
-# ifdef MSDOS
-   /* MS DOS. */
-#  define TAB_EXT	"_tab"
-#  define OUTPUT_EXT	".out"
-# else /* ! MSDOS */
-  /* Standard. */
-#  define TAB_EXT	".tab"
-#  define OUTPUT_EXT	".output"
-# endif /* ! MSDOS */
-#endif /* ! VMS */
+#ifndef OUTPUT_EXT
+# define OUTPUT_EXT ".output"
+#endif
+
+#ifndef TAB_EXT
+# define TAB_EXT ".tab"
+#endif
 
 #ifndef DEFAULT_TMPDIR
 # define DEFAULT_TMPDIR "/tmp"