spin: Update to most recent version. (thanks Ori_B)

from Ori_B:

There were a small number of changes needed from the tarball
on spinroot.org:

  - The mkfile needed to be updated
  - Memory.h needed to not be included
  - It needed to invoke /bin/cpp instead of gcc -E
  - It depended on `yychar`, which our yacc doesn't
    provide.

I'm still figuring out how to use spin, but it seems to do
the right thing when testing a few of the examples:

	% cd $home/src/Spin/Examples/
	% spin -a peterson.pml
	% pcc pan.c -D_POSIX_SOURCE
	% ./6.out

	(Spin Version 6.4.7 -- 19 August 2017)
		+ Partial Order Reduction

	Full statespace search for:
		never claim         	- (none specified)
		assertion violations	+
		acceptance   cycles 	- (not selected)
		invalid end states	+

	State-vector 32 byte, depth reached 24, errors: 0
	40 states, stored
	27 states, matched
	67 transitions (= stored+matched)
		0 atomic steps
	hash conflicts:         0 (resolved)

	Stats on memory usage (in Megabytes):
	0.002	equivalent memory usage for states (stored*(State-vector + overhead))
	0.292	actual memory usage for states
	128.000	memory used for hash table (-w24)
	0.534	memory used for DFS stack (-m10000)
	128.730	total actual memory usage


	unreached in proctype user
		/tmp/Spin/Examples/peterson.pml:20, state 10, "-end-"
		(1 of 10 states)

	pan: elapsed time 1.25 seconds
	pan: rate        32 states/second

1 files changed, 21 insertions, 14 deletions

diff --git a/sys/src/cmd/spin/tl_cache.c b/sys/src/cmd/spin/tl_cache.c
index fc902dcb0..1cf027353 100644
--- a/sys/src/cmd/spin/tl_cache.c
+++ b/sys/src/cmd/spin/tl_cache.c
@@ -1,16 +1,13 @@
 /***** tl_spin: tl_cache.c *****/
 
-/* Copyright (c) 1995-2003 by Lucent Technologies, Bell Laboratories.     */
-/* All Rights Reserved.  This software is for educational purposes only.  */
-/* No guarantee whatsoever is expressed or implied by the distribution of */
-/* this code.  Permission is given to distribute this code provided that  */
-/* this introductory message is not removed and no monies are exchanged.  */
-/* Software written by Gerard J. Holzmann.  For tool documentation see:   */
-/*             http://spinroot.com/                                       */
-/* Send all bug-reports and/or questions to: bugs@spinroot.com            */
-
-/* Based on the translation algorithm by Gerth, Peled, Vardi, and Wolper, */
-/* presented at the PSTV Conference, held in 1995, Warsaw, Poland 1995.   */
+/*
+ * This file is part of the public release of Spin. It is subject to the
+ * terms in the LICENSE file that is included in this source directory.
+ * Tool documentation is available at http://spinroot.com
+ *
+ * Based on the translation algorithm by Gerth, Peled, Vardi, and Wolper,
+ * presented at the PSTV Conference, held in 1995, Warsaw, Poland 1995.
+ */
 
 #include "tl.h"
 
@@ -24,9 +21,18 @@ typedef struct Cache {
 static Cache	*stored = (Cache *) 0;
 static unsigned long	Caches, CacheHits;
 
-static int	ismatch(Node *, Node *);
+static int ismatch(Node *, Node *);
+static int sameform(Node *, Node *);
+
 extern void fatal(char *, char *);
-int	sameform(Node *, Node *);
+
+void
+ini_cache(void)
+{
+	stored = (Cache *) 0;
+	Caches = 0;
+	CacheHits = 0;
+}
 
 #if 0
 void
@@ -182,7 +188,7 @@ sametrees(int ntyp, Node *a, Node *b)
 	return all_lfts(ntyp, b, a);
 }
 
-int	/* a better isequal() */
+static int	/* a better isequal() */
 sameform(Node *a, Node *b)
 {
 	if (!a && !b) return 1;
@@ -206,6 +212,7 @@ sameform(Node *a, Node *b)
 #ifdef NXT
 	case NEXT:
 #endif
+	case CEXPR:
 		return sameform(a->lft, b->lft);
 	case U_OPER:
 	case V_OPER: