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.

Two mechanisms are implemented to make it possible to store pointers
in shared memory on Windows, in particular on Windows Vista and later
versions with ASLR:

- The ngx_shm_remap() function added to allow remapping of a shared memory
  zone to the address originally used for it in the master process.  While
  important, it doesn't solve the problem by itself as in many cases it's
  not possible to use the address because of conflicts with other
  allocations.

- We now create mappings at the same address in all processes by starting
  mappings at predefined addresses normally unused by newborn processes.

These two mechanisms combined allow to use shared memory on Windows
almost without problems, including reloads.

Based on the patch by Sergey Brester:
http://mailman.nginx.org/pipermail/nginx-devel/2015-April/006836.html

It was only needed by the just removed rtsig module.