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.