add support for the SUA layer (interix; windows)

add support for the SUA layer (interix; windows)

* add required build options to Makefile.
* introduce new NO_INTTYPES_H for systems lacking inttypes; code
  includes stdint.h instead, if this is set.
* introduce new NO_SYS_POLL_H for systems lacking sys/poll.h; code
  includes poll.h instead, if this is set.
* introduce NO_INITGROUPS. initgroups() call is simply omitted.

Signed-off-by: Markus Duft <mduft@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>