build: link against Threads::Threads
build: link against Threads::Threads

we should pass -pthread to compiler and linker, if pthread functions,
like pthread_join() and pthread_setname_np(), are used.
for cmake 3.1 and up, we need to link against Threads::Threads, see
https://cmake.org/cmake/help/v3.1/module/FindThreads.html

Signed-off-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@redhat.com>
Message-Id: <5c4c0a85f6038fa0eedefb09028c9a94e438895d@gmail.com>

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