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/local.mk

@@ -0,0 +1,32 @@
+## Copyright (C) 2018 Free Software Foundation, Inc.
+##
+## This program is free software: you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+lexcalcdir = $(docdir)/%D%
+
+## ------ ##
+## Calc.  ##
+## ------ ##
+
+check_PROGRAMS += %D%/lexcalc
+TESTS += %D%/lexcalc.test
+EXTRA_DIST += %D%/lexcalc.test
+%C%_lexcalc_SOURCES = %D%/parse.y %D%/parse.h %D%/scan.l
+
+# Don't use gnulib's system headers.
+%C%_lexcalc_CPPFLAGS = -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
+
+dist_lexcalc_DATA = %D%/parse.y %D%/scan.l %D%/Makefile %D%/README.md
+CLEANFILES += %D%/lexcalc %D%/*.o %D%/parse.c %D%/scan.c
+CLEANDIRS += %D%/*.dSYM