make: specify C++ standard version in CXXFLAGS

The standard is now specified explicitly.  The chosen standard, c++11,
is the latest supported by gcc-4.8.5, which is the oldest compiler still
in use on some systems (centos 7).

Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Change-Id: I7ba84278b28e6d96f43b52014c358a74902188b9
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9597


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>