cmake: add a seastar_supports_flag function
cmake: add a seastar_supports_flag function

This patch implements some review feedback on my previous patch fixing
the clang build. In particular, it adds a seastar_supports_flag
function.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20190425170840.9791-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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