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.