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.

tests/regression.at

@@ -1367,11 +1367,11 @@ AT_DATA_GRAMMAR([input.y],
      large enough to overflow size_t.  */
   #define YYSIZE_T unsigned char
 
-  /* Bring in malloc and set _STDLIB_H so yacc.c doesn't try to
+  /* Bring in malloc and set EXIT_SUCCESS so yacc.c doesn't try to
      provide a malloc prototype using our YYSIZE_T.  */
   #include <stdlib.h>
-  #ifndef _STDLIB_H
-  # define _STDLIB_H 1
+  #ifndef EXIT_SUCCESS
+  # define EXIT_SUCCESS 0
   #endif
 
   /* Max depth is usually much smaller than YYSTACK_ALLOC_MAXIMUM, and