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/derivation.c

@@ -167,7 +167,7 @@ derivation_print_impl (const derivation *deriv, FILE *f,
   else if (deriv == &d_dot)
     {
       begin_use_class ("cex-dot", f);
-      fputs ("•", f);
+      print_dot (f);
       end_use_class ("cex-dot", f);
     }
   else // leaf.

src/gram.c

@@ -67,7 +67,8 @@ item_print (item_number *item, rule const *previous_rule, FILE *out)
 
   for (item_number *sp = r->rhs; sp < item; sp++)
     fprintf (out, " %s", symbols[*sp]->tag);
-  fputs (" .", out);
+  putc (' ', out);
+  print_dot (out);
   if (0 <= *r->rhs)
     for (item_number *sp = item; 0 <= *sp; ++sp)
       fprintf (out, " %s", symbols[*sp]->tag);

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);
 

src/local.mk

@@ -133,10 +133,15 @@ src_bison_LDADD =                               \
   $(ISNANL_LIBM)                                \
   $(LDEXPL_LIBM)                                \
   $(LDEXP_LIBM)                                 \
-  $(LIBINTL)                                    \
   $(LIBTHREAD)                                  \
   $(LIB_CLOCK_GETTIME)                          \
   $(LIB_GETHRXTIME)                             \
+  $(LIB_HARD_LOCALE)                            \
+  $(LIB_MBRTOWC)                                \
+  $(LIB_SETLOCALE_NULL)                         \
+  $(LIBICONV)                                   \
+  $(LIBINTL)                                    \
+  $(LIBREADLINE)                                \
   $(LIBTEXTSTYLE)