don't define _FILE_OFFSET_BITS on FreeBSD

[guijan]

_FILE_OFFSET_BITS is a glibc extension and doesn't exist on FreeBSD. Don't try to use it on FreeBSD.

Ref: https://web.archive.org/web/20240610031354/https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html#index-_005fFILE_005fOFFSET_005fBITS