Fix portability problem on OpenBSD 4.7.

Fix portability problem on OpenBSD 4.7.

Jim Meyering reported this in
<http://lists.gnu.org/archive/html/bug-bison/2010-10/msg00007.html>.
* data/yacc.c: Use EXIT_SUCCESS as a witness for stdlib.h,
not _STDLIB_H.  EXIT_SUCCESS has been defined by the standard
for quite some time.
* src/parse-gram.c, src/parse-gram.h: Regenerate.
* tests/regression.at: Tamper with the renamed witness.