don't know what they do on windows of if they even have the same problem concerning fopen.

On *nix they _FILE_OFFSET_BITS changes the off_t type into a 64 bit type, _LARGEFILE_SOURCE changes functions to their 64 bit equivalents where possible via macros (e.g. lseek -> lseek64, fopen ->fopen64, etc).
On 64 bit systems they do nothing as LFS mode is default.
I'll add a comment.