examples: add a simple Flex+Bison example in C

Suggested by Askar Safin.
http://lists.gnu.org/archive/html/bug-bison/2018-12/msg00003.html

* examples/c/lexcalc/Makefile, examples/c/lexcalc/README.md,
* examples/c/lexcalc/lexcalc.test, examples/c/lexcalc/local.mk,
* examples/c/lexcalc/parse.y, examples/c/lexcalc/scan.l:
New.

examples/c/lexcalc/Makefile

@@ -0,0 +1,34 @@
+# This Makefile is designed to be simple and readable.  It does not
+# aim at portability.  It requires GNU Make.
+
+BASE = lexcalc
+BISON = bison
+FLEX = flex
+XSLTPROC = xsltproc
+
+all: $(BASE)
+
+%.c %.h %.xml %.gv: %.y
+	$(BISON) $(BISONFLAGS) --defines --xml --graph=$*.gv -o $*.c $<
+
+%.c: %.l
+	$(FLEX) $(FLEXFLAGS) -o$@ $<
+
+scan.o: parse.h
+lexcalc: parse.o scan.o
+	$(CC) $(CFLAGS) -o $@ $^
+
+run: $(BASE)
+	@echo "Type arithmetic expressions.  Quit with ctrl-d."
+	./$<
+
+html: parse.html
+%.html: %.xml
+	$(XSLTPROC) $(XSLTPROCFLAGS) -o $@ $$($(BISON) --print-datadir)/xslt/xml2xhtml.xsl $<
+
+CLEANFILES =						\
+  $(BASE) *.o						\
+  parse.[ch] parse.output parse.xml parse.html parse.gv	\
+  scan.c
+clean:
+	rm -f $(CLEANFILES)