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.

tests/atlocal.in

@@ -133,6 +133,7 @@ fi
 : ${PERL='@PERL@'}
 
 # Use simple quotes (lib/quote.c).
+# We have an LC_ALL=C pushed onto us via maint.mk.
 LC_CTYPE=C
 export LC_CTYPE