Add rationale comments for conditional yyparse redefinition

lib/hx509/sel.h

@@ -67,6 +67,20 @@ struct hx_expr_input {
 
 extern struct hx_expr_input _hx509_expr_input;
 
+/*
+ * 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.  The below
+ * logic gets us out of the way when the job has already been taken care of by
+ * the parser-generator.
+ */
 #if !defined(yylex)
 #define yylex   _hx509_sel_yylex
 #define yywrap  _hx509_sel_yywrap