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.

.travis.yml

@@ -29,12 +29,13 @@ jobs:
         clone: true
       dist: bionic
       script:
-        - sudo apt-get install -qq autoconf automake autopoint flex gettext graphviz help2man m4 texinfo
+        - sudo apt-get install -qq autoconf automake autopoint flex gettext gperf graphviz help2man m4 texinfo
         - autoconf --version
         - automake --version
         - autopoint --version
         - dot -V
         - gettext --version
+        - gperf --version
         - help2man --version
         - makeinfo --version
         - m4 --version