Fix strange %define locations for default values.

Reported by Akim Demaille at
<http://lists.gnu.org/archive/html/bug-bison/2007-12/msg00001.html>
and discussed again starting at
<http://lists.gnu.org/archive/html/bison-patches/2008-11/msg00102.html>.
* data/bison.m4 (b4_percent_define_default): Leave syncline blank
because location information is bogus.
Use angle brackets to delimit fake file name because square brackets
look like over-quoted m4.  Choose a better fake file name.
Use negative line numbers.
* src/muscle_tab.c (muscle_percent_define_default): Likewise.
* src/location.c (location_print): If line for a boundary is negative,
only print that boundary's file name.
* src/location.h: Document that.
* tests/skeletons.at (%define Boolean variables: invalid skeleton
defaults): Update output.

tests/skeletons.at

@@ -1,5 +1,5 @@
 # Checking skeleton support.                     -*- Autotest -*-
-# Copyright (C) 2007 Free Software Foundation, Inc.
+# Copyright (C) 2007, 2009 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
@@ -159,7 +159,7 @@ start: ;
 ]])
 
 AT_BISON_CHECK([[input.y]], [[1]], [[]],
-[[[Bison:b4_percent_define_default]:1.0: invalid value for %define Boolean variable `foo'
+[[<skeleton default value>: invalid value for %define Boolean variable `foo'
 ]])
 
 AT_CLEANUP