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/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)