Don't #define yyparse when using bison/flex

With bison/flex, the more modern way to allow multiple yacc/lex
grammars to be linked into a single executable is to use the

        bison: -p, --name-prefix=,PREFIX/, -Dapi.prefix=PREFIX
        flex: -Pprefix, --prefix=STRING

options, these take care of renaming all the machine-generated
global entry points, some of which are new.  When these options are
used "yylex", "yyparse", ... are already defined and our (potentially
incomplete) attempt to do the same conflicts with the "right" new
way to handle this.  This patch gets us out of the way when the job
has already been taken care of by the parser-generator.

Originally by Christos Zoulas.

lib/hx509/sel.h

@@ -67,14 +67,18 @@ struct hx_expr_input {
 
 extern struct hx_expr_input _hx509_expr_input;
 
-#define yyparse _hx509_sel_yyparse
+#if !defined(yylex)
 #define yylex   _hx509_sel_yylex
+#define yywrap  _hx509_sel_yywrap
+#endif
+#if !defined(yyparse)
+#define yyparse _hx509_sel_yyparse
 #define yyerror _hx509_sel_yyerror
 #define yylval  _hx509_sel_yylval
 #define yychar  _hx509_sel_yychar
 #define yydebug _hx509_sel_yydebug
 #define yynerrs _hx509_sel_yynerrs
-#define yywrap  _hx509_sel_yywrap
+#endif
 
 int  _hx509_sel_yyparse(void);
 int  _hx509_sel_yylex(void);