The auto/feature call above is enough to set NGX_HAVE_SENDFILE.

It appeared that the NGX_HAVE_AIO_SENDFILE macro was defined regardless of
the "--with-file-aio" configure option and the NGX_HAVE_FILE_AIO macro.

Now they are related.

Additionally, fixed one macro.

Since Linux 2.6.17, epoll is able to report about peer half-closed connection
using special EPOLLRDHUP flag on a read event.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.

It was introduced in Linux 2.6.39, glibc 2.14 and allows to obtain
file descriptors without actually opening files.  Thus made it possible
to traverse path with openat() syscalls without the need to have read
permissions for path components.  It is effectively emulates O_SEARCH
which is missing on Linux.

O_PATH is used in combination with O_RDONLY.  The last one is ignored
if O_PATH is used, but it allows nginx to not fail when it was built on
modern system (i.e. glibc 2.14+) and run with a kernel older than 2.6.39.
Then O_PATH is unknown to the kernel and ignored, while O_RDONLY is used.

Sadly, fstat() is not working with O_PATH descriptors till Linux 3.6.
As a workaround we fallback to fstatat() with the AT_EMPTY_PATH flag
that was introduced at the same time as O_PATH.

HP-UX needs _HPUX_ALT_XOPEN_SOCKET_API to be defined to be able to
use various POSIX versions of networking functions.  Notably sendmsg()
resulted in "sendmsg() failed (9: Bad file number)" alerts without it.

See xopen_networking(7) for more details.

This fixes alignment problems observerd on ARMs, and likely also needed
for MIPSes.  Unless we know alignment is not required just assume we
need 16, which appears to be safe default for all architectures.

See here for details:
http://mailman.nginx.org/pipermail/nginx/2012-June/034139.html