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.

tests/bison.in

@@ -40,8 +40,9 @@ do
       # post-processes the synclines on y.tab.c itself.  Don't let it
       # do it.  Besides, it leaves "parse-gram.y" as the source,
       # dropping the src/ part.
-      $PERL -pi -e 's{"y\.tab\.}{"src/parse-gram.}g;'    \
-                -e 's{".*/(parse-gram\.y)"}{"src/$1"}g;' \
+      $PERL -pi -e 's{"y\.tab\.}{"src/parse-gram.}g;'           \
+                -e 's{".*/(parse-gram\.y)"}{"src/$1"}g;'        \
+                -e 's{GRAM_Y_TAB_H}{GRAM_SRC_PARSE_GRAM_H}g;'        \
            y.tab.[ch]
     fi
     ;;