Define CLI11_HAS_FILESYSTEM=0 for GCC 8
Define CLI11_HAS_FILESYSTEM=0 for GCC 8

As outlined on https://github.com/CLIUtils/CLI11, filesystem library is
separate from the standard library on GCC 8. CLI11 uses the filesystem
library only for the path type checks, which are not used by cleos, so
it's safe to switch to the fallback implementation provided by CLI11.

EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos