Work around porting bugs reported by Dieter in

Work around porting bugs reported by Dieter in
<http://lists.gnu.org/archive/html/bug-bison/2006-01/msg00049.html>.
* lib/getopt.c (_NOPROTO): Remove; no longer needed.
Include <stdlib.h> and <unistd.h> in all environments; it's safe now.
Include "getopt.h" first, to check interface.
(getenv): Declare only if defined HAVE_DECL_GETENV &&
!HAVE_DECL_GETENV.
* lib/strndup.c [!_LIBC]: Include "strndup.h" to get prototype.
(__strndup): Revert to K&R-style function dfns, the glibc style.
* lib/strnlen.c: Don't claim it's taken from glibc; it's not.
(strnlen, __strnlen): Remove #defines and #undefs; not needed.
Include strnlen.h first, to get prototype properly.
(strnlen): Renamed from __strnlen.
Remove weak alias.
* m4/getopt.m4 (gl_PREREQ_GETOPT): Check for getenv decl.