bizarre ifdef logic in generated parser

[Aharon Robbins]

Greetings.

I note the following:

        # if YYSTACK_USE_ALLOCA
        #  define YYSTACK_ALLOC alloca
        # else
--->    #  ifndef YYSTACK_USE_ALLOCA
        #   if defined (alloca) || defined (_ALLOCA_H)
        #    define YYSTACK_ALLOC alloca
        #   else
        #    ifdef __GNUC__
        #     define YYSTACK_ALLOC __builtin_alloca
        #    endif
        #   endif
--->    #  endif
        # endif

It seems that the marked test can be omitted, given that it's already
in the else part of `if YYSTACK_USE_ALLOCA', we know it's not
defined.

Arnold