With this change it's no longer needed to pass -D_GNU_SOURCE manually,
and -D_FILE_OFFSET_BITS=64 is set to use 64-bit off_t.

Note that nginx currently fails to work properly with master process
enabled on GNU Hurd, as fcntl(F_SETOWN) returns EOPNOTSUPP for sockets
as of GNU Hurd 0.6.  Additionally, our strerror() preloading doesn't
work well with GNU Hurd, as it uses large numbers for most errors.

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

*) Darwin sendfile() support

style fix