Configure: GNU Hurd properly recognized. 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.