fix posix_spawn test for MacOS X

[Bruno Haible]

On MacOS X the posix_spawn currently fails, although the system function
behaves perfectly fine - just differently than the glibc implementation.
This fixes it.

2008-10-20  Bruno Haible  <address@hidden>

        * m4/posix_spawn.m4 (gl_POSIX_SPAWN_WORKS): Make the check on MacOS X.

*** m4/posix_spawn.m4.orig      2008-10-20 13:07:47.000000000 +0200
--- m4/posix_spawn.m4   2008-10-20 13:07:36.000000000 +0200
***************
*** 1,4 ****
! # posix_spawn.m4 serial 3
  dnl Copyright (C) 2008 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
--- 1,4 ----
! # posix_spawn.m4 serial 4
  dnl Copyright (C) 2008 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
***************
*** 159,167 ****
        if (attrs_allocated)
        posix_spawnattr_destroy (&attrs);
        sigprocmask (SIG_UNBLOCK, &fatal_signal_set, NULL);
!       errno = err;
!       perror ("subprocess failed");
!       exit (1);
      }
    posix_spawn_file_actions_destroy (&actions);
    posix_spawnattr_destroy (&attrs);
--- 159,172 ----
        if (attrs_allocated)
        posix_spawnattr_destroy (&attrs);
        sigprocmask (SIG_UNBLOCK, &fatal_signal_set, NULL);
!       if (err == ENOENT)
!       return 0;
!       else
!       {
!         errno = err;
!         perror ("subprocess failed");
!         exit (1);
!       }
      }
    posix_spawn_file_actions_destroy (&actions);
    posix_spawnattr_destroy (&attrs);