examples: add an example with a reentrant parser in Flex+Bison

Suggested by Eric S. Raymond.
https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00066.html

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

examples/c/reccalc/README.md

@@ -0,0 +1,45 @@
+# reccalc - recursive calculator with Flex and Bison
+
+In this example the generated parser is pure and reentrant: it can be used
+concurrently in different threads, or recursively.  As a proof of this
+reentrancy, expressions in parenthesis are tokenized as strings, and then
+recursively parsed from the parser:
+
+```
+exp: STR
+  {
+    result r = parse_string ($1);
+    free ($1);
+    if (r.nerrs)
+      {
+        res->nerrs += r.nerrs;
+        YYERROR;
+      }
+    else
+      $$ = r.value;
+  }
+```
+
+<!---
+Local Variables:
+fill-column: 76
+ispell-dictionary: "american"
+End:
+
+Copyright (C) 2018-2019 Free Software Foundation, Inc.
+
+This file is part of Bison, the GNU Compiler Compiler.
+
+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/>.
+--->