add -D_FILE_OFFSET_BITS=64 to fix 2gb file size limit

The -D_FILE_OFFSET_BITS=64 define is missing; required to write files > 2gb.
http://www.raspberrypi.org/phpBB3/viewtopic.php?p=359291#p359291
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=43&t=59089