examples: bistromathic: demonstrate internationalization

Currently it was only using stubs.  Let's actually translate the
strings using gettext.

* examples/c/bistromathic/local.mk: Define LOCALEDIR, BISON_LOCALEDIR
and link with libintl.
* examples/c/bistromathic/parse.y: Use them.
Remove useless includes.
Take ENABLE_NLS into account.
(error_format_string): New.
(yyreport_syntax_error): Rewrite to rely on a format string, which is
more appropriate for internationalization.
* examples/c/bistromathic/Makefile: We no longer use Flex.
We need readline and intl.

* doc/bison.texi: Point to bistromathic for a better option for
internationalization.
* po/POTFILES.in: Add bistromathic.

examples/c/bistromathic/local.mk

@@ -26,8 +26,11 @@ nodist_%C%_bistromathic_SOURCES = %D%/parse.y %D%/parse.h
 %D%/parse.c: $(dependencies)
 
 # Don't use gnulib's system headers.
-%C%_bistromathic_CPPFLAGS = -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
-%C%_bistromathic_LDADD = -lm -lreadline
+%C%_bistromathic_CPPFLAGS =			\
+  -DBISON_LOCALEDIR='"$(localdir)"'		\
+  -DLOCALEDIR='"$(localdir)"'			\
+  -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
+%C%_bistromathic_LDADD = -lm -lreadline $(LIBINTL)
 
 dist_bistromathic_DATA = %D%/parse.y %D%/Makefile %D%/README.md
 CLEANFILES += %D%/parse.[ch] %D%/parse.output