Change %pure_parser to %pure-parser in cppBison.yxx (%pure_parser is very very deprecated)

dtool/src/cppparser/cppBison.yxx

@@ -194,7 +194,7 @@ pop_struct() {
 /* This is a bison-specific declaration to enable recursive calls to
    yyparse().  It changes the calling sequence to yylex(), passing
    pointers to the current yylval and yylloc. */
-%pure_parser
+%pure-parser
 
 %token <u.real> REAL
 %token <u.integer> INTEGER