Fix incompatibilities between MinGW.org and MinGW64 headers

Fix incompatibilities between MinGW.org and MinGW64 headers

Problems were reported by ׃scar Fuentes in

  http://lists.gnu.org/archive/html/emacs-devel/2013-03/msg00611.html.

 nt/inc/ms-w32.h (struct timespec): Don't define if
 _TIMESPEC_DEFINED is already defined.
 (sigset_t) [!_POSIX]: Typedef for MinGW64.
 (_WIN32_WINNT, WIN32_LEAN_AND_MEAN): Move definitions before
 including the first system header, to avoid redefinition if some
 system header defines a default value.
 nt/inc/sys/time.h (struct itimerval): Don't define if
 _TIMESPEC_DEFINED is already defined.