bistromathic: beware of portability of readline

Don't try to build bistromathic if we don't have readline.
Reported by Bruno Haible.
https://lists.gnu.org/r/bug-bison/2020-05/msg00028.html

* configure.ac (ENABLE_BISTROMATHIC): New.
* examples/c/bistromathic/local.mk: Use it.
* examples/c/bistromathic/bistromathic.test: Exit 77 for skip.

examples/c/bistromathic/bistromathic.test

@@ -35,7 +35,7 @@ if ! echo '1-1' | prog | grep '>' >/dev/null; then
 elif ! echo '1-1' | prog | grep '1-1' >/dev/null; then
     # OpenBSD 6.5.  I don't want to spend time on this.
     echo "SKIP: this is not the GNU Readline we expect"
-    exit 0
+    exit 77
 fi
 
 

examples/c/bistromathic/local.mk

@@ -19,19 +19,22 @@ bistromathicdir = $(docdir)/%D%
 ## Bistromathics.  ##
 ## --------------- ##
 
-check_PROGRAMS += %D%/bistromathic
-TESTS += %D%/bistromathic.test
-EXTRA_DIST += %D%/bistromathic.test
-nodist_%C%_bistromathic_SOURCES = %D%/parse.y %D%/parse.h
 %D%/parse.c: $(dependencies)
 
-# Don't use gnulib's system headers.
-%C%_bistromathic_CPPFLAGS =			\
-  -DBISON_LOCALEDIR='"$(localdir)"'		\
-  -DLOCALEDIR='"$(localdir)"'			\
-  -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
-%C%_bistromathic_LDADD = -lm -lreadline $(LIBINTL)
+if ENABLE_BISTROMATHIC
+  check_PROGRAMS += %D%/bistromathic
+  TESTS += %D%/bistromathic.test
+  nodist_%C%_bistromathic_SOURCES = %D%/parse.y
 
+  # Don't use gnulib's system headers.
+  %C%_bistromathic_CPPFLAGS =			\
+    -DBISON_LOCALEDIR='"$(localdir)"'		\
+    -DLOCALEDIR='"$(localdir)"'			\
+    -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
+  %C%_bistromathic_LDADD = -lm $(LIBREADLINE) $(LIBINTL)
+endif
+
+EXTRA_DIST += %D%/bistromathic.test
 dist_bistromathic_DATA = %D%/parse.y %D%/Makefile %D%/README.md
 CLEANFILES += %D%/parse.[ch] %D%/parse.output
 CLEANDIRS += %D%/*.dSYM