poll-h: Create poll.h on all platforms.

poll-h: Create poll.h on all platforms.

* lib/poll.in.h: Use double-inclusion guard. Don't define POLL*,
struct pollfd, nfds_t, INFTIM when the system has <poll.h>.
* m4/poll_h.m4 (gl_POLL_H): Set HAVE_POLL_H. Invoke
gl_CHECK_NEXT_HEADERS. Don't set POLL_H.
(gl_REPLACE_POLL_H): Don't set POLL_H.
(gl_POLL_H_DEFAULTS): Don't initialize POLL_H.
* modules/poll-h (Depends-on): Add include_next.
(Makefile.am): Create poll.h unconditionally. Substitute also
HAVE_POLL_H, INCLUDE_NEXT, PRAGMA_SYSTEM_HEADER, NEXT_POLL_H.