sclang/scsynth: use std::thread if available

Signed-off-by: Tim Blechmann <tim@klingt.org>
cmake build system: avoid inclusion of obsolete boost-lockfree folder

boost.lockfree is now part of boost

Signed-off-by: Tim Blechmann <tim@klingt.org>
cmake build system: sclang still requires libboost_thread for the threadpool

Signed-off-by: Tim Blechmann <tim@klingt.org>