seastar does not build any 3rd party libraries. but we do. currently, we
build DPDK/SPDK when they are enabled for the messenger backend and
bluestore backend.
in this change, the same function is reused for preparing the DPDK
libraries if `Seastar_DPDK` is enabled. probably we should outsource
this work to install-deps.sh in future, but before that, let's keep
this as a builtin "feature".