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/parse.y

@@ -0,0 +1,95 @@
+// Prologue (directives).
+%expect 0
+
+// Emitted in the header file, after the definition of YYSTYPE.
+%code provides
+{
+  // Tell Flex the expected prototype of yylex.
+#define YY_DECL                                 \
+  enum yytokentype yylex (YYSTYPE* yylval, int *nerrs)
+  YY_DECL;
+
+  void yyerror (int *nerrs, const char *msg);
+}
+
+// Emitted on top of the implementation file.
+%code top
+{
+#include <stdio.h>  /* printf. */
+#include <stdlib.h> /* getenv. */
+}
+
+%define api.pure full
+%define api.token.prefix {TOK_}
+%define api.value.type union
+%define parse.error verbose
+%define parse.trace
+ // Error count, exchanged between main, yyparse and yylex.
+%param {int *nerrs}
+
+%token
+  PLUS   "+"
+  MINUS  "-"
+  STAR   "*"
+  SLASH  "/"
+  LPAREN "("
+  RPAREN ")"
+  EOL    "end-of-line"
+  EOF 0  "end-of-file"
+;
+
+%token <int> NUM "number"
+%type <int> exp line
+%printer { fprintf (yyo, "%d", $$); } <int>
+
+// Precedence (from lowest to highest) and associativity.
+%left "+" "-"
+%left "*" "/"
+
+%%
+// Rules.
+input:
+  %empty
+| input line  { printf ("%d\n", $line); }
+;
+
+line:
+  exp EOL   { $$ = $1; }
+| error EOL { yyerrok; }
+;
+
+exp:
+  exp "+" exp   { $$ = $1 + $3;  }
+| exp "-" exp   { $$ = $1 - $3;  }
+| exp "*" exp   { $$ = $1 * $3;  }
+| exp "/" exp
+  {
+    if ($3 == 0)
+      {
+        yyerror (nerrs, "invalid division by zero");
+        YYERROR;
+      }
+    else
+      $$ = $1 / $3;
+  }
+| "(" exp ")"   { $$ = $2;       }
+| NUM           { $$ = $1; }
+;
+%%
+// Epilogue (C code).
+void yyerror(int *nerrs, const char *msg)
+{
+  fprintf (stderr, "%s\n", msg);
+  ++nerrs;
+}
+
+int main (void)
+{
+  int nerrs = 0;
+  // Enable parser runtime debugging.
+  if (!!getenv ("YYDEBUG"))
+    yydebug = 1;
+  yyparse (&nerrs);
+  // Exit on failure if there were errors.
+  return !!nerrs;
+}