Unmangle the file IO by using AC_SYS_LARGEFILE and AC_FUNC_FSEEKO

There are parts of the code that cares too much about LFS. Using AC_SYS_LARGEFILE with AC_FUNC_FSEEKO should handle the 64-bit IO (mostly) automatically.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information