Previously we used _fseeki64() and _ftelli64() for 64-bit file
 access on Windows, but this requires MSVCRT80 (which requires MSVC
 2005 and is not shipped on XP by default).
Reimplement these functions using the C89 fgetpos() and fsetpos(),
 relying on the MSVCRT/minw32-specific definition of fpos_t.