gnulib: add module sys_wait, for compilation on MinGW

Also, upgrade to gnulib 4af1990a0902f5914f582bade9d1aa843a291f5a.

* lib/sys_wait.in.h, m4/sys_wait_h.m4: New files.
* m4/sys_stat_h.m4, m4/stdlib_h.m4, m4/gnulib-comp.m4,
m4/gnulib-cache.m4, lib/stdlib.in.h, lib/Makefile.am: Update.

m4/sys_wait_h.m4

@@ -0,0 +1,36 @@
+# sys_wait_h.m4 serial 6
+dnl Copyright (C) 2008-2012 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_SYS_WAIT_H],
+[
+  AC_REQUIRE([gl_SYS_WAIT_H_DEFAULTS])
+
+  dnl <sys/wait.h> is always overridden, because of GNULIB_POSIXCHECK.
+  gl_CHECK_NEXT_HEADERS([sys/wait.h])
+
+  dnl Ensure the type pid_t gets defined.
+  AC_REQUIRE([AC_TYPE_PID_T])
+
+  dnl Check for declarations of anything we want to poison if the
+  dnl corresponding gnulib module is not in use.
+  gl_WARN_ON_USE_PREPARE([[#include <sys/wait.h>]],
+    [waitpid])
+])
+
+AC_DEFUN([gl_SYS_WAIT_MODULE_INDICATOR],
+[
+  dnl Use AC_REQUIRE here, so that the default settings are expanded once only.
+  AC_REQUIRE([gl_SYS_WAIT_H_DEFAULTS])
+  gl_MODULE_INDICATOR_SET_VARIABLE([$1])
+  dnl Define it also as a C macro, for the benefit of the unit tests.
+  gl_MODULE_INDICATOR_FOR_TESTS([$1])
+])
+
+AC_DEFUN([gl_SYS_WAIT_H_DEFAULTS],
+[
+  GNULIB_WAITPID=0; AC_SUBST([GNULIB_WAITPID])
+  dnl Assume proper GNU behavior unless another module says otherwise.
+])