With bison 2.7 YYDEBUG will be set to 0 if it was not set, then #ifdef
YYDEBUG returns true, but bison 2.7 treets YYDEBUG 0 as not set.

This fixes the following error:
obj/main.o: In function `main':
main.c:(.text.startup+0xaf): undefined reference to `yydebug'
collect2: error: ld returned 1 exit status

This closes #13093.

SVN-Revision: 35830