yydebug is defined by yacc itself when -D is specified.
defining yydebug yourself will lead to errors.

Reference: /n/sources/patch/maybe/yacc-yydebug
Date: Thu Apr 25 23:17:46 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/man/1/yacc	Thu Apr 25 23:17:16 2013
+++ /sys/man/1/yacc	Thu Apr 25 23:17:15 2013
@@ -39,9 +39,6 @@
 Create file
 .BR y.debug ,
 containing diagnostic messages.
-To incorporate them in the parser, compile it with preprocessor symbol
-.B yydebug
-defined.
 The amount of 
 diagnostic output from the parser is regulated by
 value