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.

lib/.gitignore

@@ -150,6 +150,23 @@
 /hard-locale.h
 /hash.c
 /hash.h
+/iconv.c
+/iconv.h
+/iconv.in.h
+/iconv_close.c
+/iconv_open-aix.gperf
+/iconv_open-aix.h
+/iconv_open-hpux.gperf
+/iconv_open-hpux.h
+/iconv_open-irix.gperf
+/iconv_open-irix.h
+/iconv_open-osf.gperf
+/iconv_open-osf.h
+/iconv_open-solaris.gperf
+/iconv_open-solaris.h
+/iconv_open-zos.gperf
+/iconv_open-zos.h
+/iconv_open.c
 /intprops.h
 /inttypes.h
 /inttypes.in.h
@@ -327,11 +344,16 @@
 /timespec.h
 /timevar.c
 /timevar.h
+/unicodeio.c
+/unicodeio.h
 /unistd--.h
 /unistd-safer.h
 /unistd.c
 /unistd.h
 /unistd.in.h
+/unistr.h
+/unistr.in.h
+/unistr/
 /unitypes.h
 /unitypes.in.h
 /uniwidth
@@ -340,6 +362,7 @@
 /unlink.c
 /unlocked-io.h
 /unsetenv.c
+/unused-parameter.h
 /vasnprintf.c
 /vasnprintf.h
 /vasprintf.c