Make the MinGW32 build compatible with MinGW runtime 4.x.

Make the MinGW32 build compatible with MinGW runtime 4.x.

 nt/inc/dirent.h (struct dirent) [__MINGW_MAJOR_VERSION >= 4]: Make
 the layout of 'struct dirent' be compatible with MinGW32 runtime
 versions 4.0 and later.
 nt/inc/ms-w32.h (__MINGW_MAJOR_VERSION, __MINGW_MINOR_VERSION)
 (__MINGW_PATCHLEVEL) [!__MINGW64_VERSION_MAJOR]: Define, if not
 defined, but not for MinGW64.
 (_USE_32BIT_TIME_T) [__MINGW64_VERSION_MAJOR]: Define, to force
 use of 32-bit time_t type.