New module 'system-posix'.

New module 'system-posix'.

* modules/system-posix: New file.
* lib/stdlib.in.h: Include <sys/wait.h> only when the 'system-posix'
module is present.
* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize
GNULIB_SYSTEM_POSIX.
* modules/stdlib (Depends-on): Remove sys_wait.
(Makefile.am): Substitute GNULIB_SYSTEM_POSIX.
* doc/posix-functions/system.texi: Mention the new module.
* doc/posix-headers/stdlib.texi: Likewise.
* tests/test-stdlib.c: If GNULIB_TEST_SYSTEM_POSIX is not defined,
define test_sys_wait_macros to a no-op.
Reported by Sam Steingold <sds@gnu.org>.