Log Message:
don't include poll.h or sys/poll.h, but instead, undefine HAVE_POLL_H
and HAVE_SYS_POLL_H if we don't have a native poll() implementation.
prevents including of libnbcompat's poll.h as well as the system's
poll.h where we should be ignoring the system's poll.h.

fix tested by Jeff Rizzo on MacOSX 10.3.3.