obstack: add NEWS entry for recent incompatible changes

* NEWS: Describe recent changes.
open, openat: document nonstandard FreeBSD, NetBSD O_NOFOLLOW errno

* doc/posix-functions/open.texi (open):
* doc/posix-functions/openat.texi (openat):
Document that these functions do not set errno to ELOOP when
a symlink is opened with O_NOFOLLOW.
linkat: wrap to handle symlinks on OS X 10.10

* m4/linkat.m4 (gl_FUNC_LINKAT): linkat() is available on Yosemite
but not usable because it doesn't support creating hardlinks
to symlinks.  Therefore add a generic test for this capability
and fallback to our emulation if linkat() fails with ENOTSUP.
linkat: don't unconditionally replace on GNU/Linux

* m4/linkat.m4 (gl_FUNC_LINKAT): The compile check for AT_SYMLINK_FOLLOW
was redundant for a few reasons.  It was present to support compiling
on new systems but running on the old narrow window of Linux 2.6.1[67].
It setup and cleaned up test files which weren't actually used.
On non __linux__ it compile tested AT_SYMLINK_FOLLOW, but that is
implicit in the following check.
obstack: port to platforms that #define __alignof__

* lib/obstack.c: Include <alignof.h> if !defined __alignof__,
not if !_LIBC.  We don't know of any platforms that #define
__alignof__, but it might be useful in tests.  Conversely,
glibc assumes GCC.
obstack: do not assume system-supplied obstack is size_t safe

* m4/obstack.m4: New file.
* modules/obstack (Files): Add it.
obstack: fix macro return values

* lib/obstack.h (obstack_next_free): Return void *.
(obstack_1grow_fast, obstack_blank_fast): Return void.
For __GNUC__ macros:
(obstack_1grow, obstack_blank): Remove now unnecessary (void) 0.
For !__GNUC__ macros:
(obstack_make_room, obstack_grow, obstack_grow0)
(obstack_ptr_grow_fast, obstack_int_grow_fast): Return void.
obstack: avoid potentially-nonportable function casts

* lib/obstack.c (CALL_CHUNKFUN, CALL_FREEFUN):
Remove, replacing with ...
(call_chunkfun, call_freefun): New static functions.
All uses changed.  Avoid potentially-nonportable casts.
(chunkfun_type, freefun_type): Remove typedefs; no longer used.
(_obstack_begin_worker): Omit last two args, since they
rely on potentially-nonportable casts.  All callers changed.
* lib/obstack.h (_OBSTACK_CAST): New macro.
Use it everywhere the old API used a potentially-nonportable cast.
The new API doesn't cast.
(struct obstack): Use unions rather than requiring
potentially-nonportable casts.
(obstack_chunkfun, obstack_freefun): Return void.
update from texinfo
autoupdate
obstack: do not reject malloc-style obstack_chunkfun, obstack_freefun

Problem reported by Alan Modra in:
http://lists.gnu.org/archive/html/bug-gnulib/2014-11/msg00007.html
* lib/obstack.h (obstack_chunkfun, obstack_freefun):
Always cast the function arg, reverting this part of the previous
change.