dmclock: pick up change to use specified C++ settings if any

dmclock: pick up change to use specified C++ settings if any

see https://github.com/ceph/dmclock/pull/73

so dmclock won't pass `-std=c++11` even if the parent project already
have `-std=c++17` in CXX_FLAGS.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit fb83b331de9636e5de3d756e490748d5bfb9c4dc)

Conflicts: dmclock
  this change is not cherry-picked from master, because master is using
  a different dmclock version. so in this change, only the minimum fix
  is included.