Hide warnings from wait

[Ralf Wildenhues]

On Cygwin, depcomp2.test failed due to this in stderr:

./configure: line 3393: wait: pid 3856 is not a child of this shell

Let's hide it, it only confuses the user.  (Posix documents that the
shell does not wait for PIDs reused by unrelated processes.)
I'm merging this patch from the resurrected sanity-sleep-fix-master
branch.

Cheers,
Ralf

    Avoid configure warnings from wait about reused PIDs.
    
    * m4/sanity.m4 (AM_SANITY_CHECK): Hide wait stderr output.
    Fixes spurious failure of depcomp2.test.

diff --git a/m4/sanity.m4 b/m4/sanity.m4
index ba97076..a9cac5c 100644
--- a/m4/sanity.m4
+++ b/m4/sanity.m4
@@ -1,13 +1,13 @@
 # Check to make sure that the build environment is sane.    -*- Autoconf -*-
 
-# Copyright (C) 1996, 1997, 2000, 2001, 2003, 2005, 2008, 2009, 2010
-# Free Software Foundation, Inc.
+# Copyright (C) 1996, 1997, 2000, 2001, 2003, 2005, 2008, 2009, 2010,
+# 2011 Free Software Foundation, Inc.
 #
 # This file is free software; the Free Software Foundation
 # gives unlimited permission to copy and/or distribute it,
 # with or without modifications, as long as this notice is preserved.
 
-# serial 7
+# serial 8
 
 # AM_SANITY_CHECK
 # ---------------
@@ -77,7 +77,8 @@ fi
 AC_CONFIG_COMMANDS_PRE(
   [AC_MSG_CHECKING([that generated files are newer than configure])
    if test -n "$am_sleep_pid"; then
-     wait $am_sleep_pid
+     # Hide warnings about reused PIDs.
+     wait $am_sleep_pid 2>/dev/null
    fi
    AC_MSG_RESULT([done])])
 rm -f conftest.file