Update `%pure-parser` to `%define api.pure` for newer `bison`; fix #4706

mrbgems/mruby-compiler/core/parse.y

@@ -1280,7 +1280,7 @@ heredoc_end(parser_state *p)
 
 %}
 
-%pure-parser
+%define api.pure
 %parse-param {parser_state *p}
 %lex-param {parser_state *p}