posix_spawn: avoid spurious message in configure output

[Bruno Haible]

On Haiku, I'm seeing this configure output:

...
checking for posix_spawn... yes
checking whether posix_spawn works... cmp: EOF on conftest.ok
no
checking for ptrdiff_t... yes
...

This patch should avoid the spurious message. 'cmp' is required by POSIX to
emit a diagnostics to stderr, even when nothing is wrong with the two argument
files: <http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cmp.html>.
Yuck.


2017-10-28 Bruno Haible <***@clisp.org>

posix_spawn: Avoid spurious message in configure output.
* m4/posix_spawn.m4 (gl_POSIX_SPAWN_WORKS): Discard stderr output from
'cmp' command.

diff --git a/m4/posix_spawn.m4 b/m4/posix_spawn.m4
index 47a6147..d7d1c86 100644
--- a/m4/posix_spawn.m4
+++ b/m4/posix_spawn.m4
@@ -1,4 +1,4 @@
-# posix_spawn.m4 serial 13
+# posix_spawn.m4 serial 14
dnl Copyright (C) 2008-2017 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
@@ -224,7 +224,7 @@ main ()
[if test -s conftest$ac_exeext \
&& ./conftest$ac_exeext > conftest.out \
&& echo 'This should be seen only once.' > conftest.ok \
- && cmp conftest.out conftest.ok > /dev/null; then
+ && cmp conftest.out conftest.ok >/dev/null 2>&1; then
gl_cv_func_posix_spawn_works=yes
else
gl_cv_func_posix_spawn_works=no