build: don't error out on preprocessor #warning
build: don't error out on preprocessor #warning

Similarly to [[deprecated]] warnings.
Message-Id: <96afa53f763c602aaf1cc5fbee64e5521608fe3b@scylladb.com>

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