examples: add a demonstration of GLR parsers in C

Based on the test case 668 (cxx-type.at:437) "GLR: Merge conflicting
parses, pure, locations".

* examples/c/glr/Makefile, examples/c/glr/README.md,
* examples/c/glr/c++-types.test, examples/c/glr/c++-types.y,
* examples/c/glr/local.mk: New.

examples/c/glr/local.mk

@@ -0,0 +1,34 @@
+## Copyright (C) 2020 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/>.
+
+glrdir = $(docdir)/%D%
+
+## ----------- ##
+## c++-types.  ##
+## ----------- ##
+
+check_PROGRAMS += %D%/c++-types
+TESTS += %D%/c++-types.test
+EXTRA_DIST += %D%/c++-types.test
+nodist_%C%_c___types_SOURCES = %D%/c++-types.y
+%D%/c++-types.c: $(dependencies)
+
+# Don't use gnulib's system headers.
+%C%_c___types_CPPFLAGS = -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
+%C%_c___types_CFLAGS = $(TEST_CFLAGS)
+
+dist_glr_DATA = %D%/c++-types.y %D%/Makefile %D%/README.md
+CLEANFILES += %D%/c++-types.c %D%/c++-types.output
+CLEANDIRS += %D%/*.dSYM