glr2.cc: the example requires Bison 3.8

This will save us from generating the position.hh file.

* src/parse-gram.y: Claim we are 3.8.
* examples/c++/glr/c++-types.yy: Require 3.8.

examples/c++/glr/c++-types.yy

@@ -18,6 +18,7 @@
 /* Simplified C++ Type and Expression Grammar.
    Written by Paul Hilfinger for Bison's test suite.  */
 
+%require "3.8"
 %glr-parser
 %skeleton "glr2.cc"
 %define parse.assert