build: enable concepts whenever they are supported by compiler
build: enable concepts whenever they are supported by compiler

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <81e0311d8590035005c57ff303e1fd803e2151fe.1529082511.git.2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar