cex: don't assume the terminal supports "•"

Use of print_unicode_char suggested by Bruno Haible.
https://lists.gnu.org/r/bug-gettext/2020-06/msg00012.html

* src/gram.h (print_dot_fallback, print_dot): New.
* src/gram.c, src/derivation.c: Use it.
* tests/counterexample.at, tests/report.at: Adjust the test suite.
* .travis.yml, README-hacking.md: Adjust.

src/gram.h

@@ -101,6 +101,10 @@
 
    Associativities are recorded similarly in SYMBOLS[I]->assoc.  */
 
+# include "system.h"
+
+# include <unicodeio.h>
+
 # include "location.h"
 # include "symtab.h"
 
@@ -213,6 +217,25 @@ typedef struct
 extern rule *rules;
 extern rule_number nrules;
 
+/* Fallback in case we can't print "•".  */
+static inline long
+print_dot_fallback (unsigned int code _GL_UNUSED,
+                    const char *msg _GL_UNUSED,
+                    void *callback_arg)
+{
+  FILE *out = (FILE *) callback_arg;
+  putc ('.', out);
+  return -1;
+}
+
+/* Print "•", the symbol used to represent a point in an item (aka, a
+   pointed rule).  */
+static inline void
+print_dot (FILE *out)
+{
+  unicode_to_mb (0x2022, fwrite_success_callback, print_dot_fallback, out);
+}
+
 /* Get the rule associated to this item.  ITEM points inside RITEM.  */
 rule const *item_rule (item_number const *item);