cmake: do not find_package(Boost ...) if Boost is a target
cmake: do not find_package(Boost ...) if Boost is a target

otherwise FindBoost.cmake will, for example, try to read the version
number from version.hpp, which is not ready yet if Boost is a target
not built yet.

Signed-off-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@redhat.com>
Message-Id: <35deac06cbb702823c1983d7e313de0ab6a7d19f@gmail.com>

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