maint: fix bison's own header guards.

maint: fix bison's own header guards.

Because I'm using a VPATH build with an absolute srcdir, I have
GRAM__USERS_AKIM_SRC_GNU_BISON_SRC_PARSE_GRAM_H.  Before, I was using
a relative srcdir, and had GRAM_______SRC_PARSE_GRAM_H (coming from
../../).  Let it be GRAM_SRC_PARSE_GRAM_H.

* tests/bison.in: Do not depend on the value of $top_srcdir for
Bison itself.
If we were to use relative paths from .c to .y, we would not have
this problem.