build: bring back SEASTAR_DEFAULT_ALLOCATOR and SEASTAR_HAVE_LZ4_COMPRESS_DEFAULT
build: bring back SEASTAR_DEFAULT_ALLOCATOR and SEASTAR_HAVE_LZ4_COMPRESS_DEFAULT

The definitions SEASTAR_DEFAULT_ALLOCATOR and
SEASTAR_HAVE_LZ4_COMPRESS_DEFAULT are used from scylla, and so have to
be public.
Message-Id: <faffeef9225258ef3679136ee997ebde688afb52@scylladb.com>

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