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.

diff --git a/src/dmclock b/src/dmclock
index acfde8e45c0715e35046b581c72772a66d2133a1..4496dbc6515db96e08660ac38883329c5009f3e9 160000 (submodule)
--- a/src/dmclock
+++ b/src/dmclock
@@ -1 +1 @@
-Subproject commit acfde8e45c0715e35046b581c72772a66d2133a1
+Subproject commit 4496dbc6515db96e08660ac38883329c5009f3e9