How to enable debug code?

Hi, there:

I met some problem when enabling debug code, here it is:

I want to debug the YACC of GDB command line. Specifically, I want
this code to work: `YYDPRINTF ((stderr, "Starting parse\n"));', which
at c-exp.c

So I've tried
1. make 'CFLAGS=-DYYDEBUG'
2. make YYDEBUG=1

Both failed. Any hint? Thanks.