Introduce a versioning mechanism, and an `About' box in all front

ends. Versioning will be done solely by Subversion revision number,
since development on these puzzles is very incremental and gradual
and there don't tend to be obvious points to place numbered
releases.

[originally from svn r5781]

puzzles.h

@@ -161,6 +161,11 @@ char *dupstr(const char *s);
  */
 void free_cfg(config_item *cfg);
 
+/*
+ * version.c
+ */
+extern char ver[];
+
 /*
  * random.c
  */