Fixed make_unique to work with C++14+ compilers

Fixed make_unique to work with C++14+ compilers

Previous solution didn't correctly import the make_unique from
C++14/1z when using the new compiler.  This ensures that OC::make_unique
will always work correctly.

Change-Id: Ibb7456f6375c5c16553bb76c71646ec911190c0f
Signed-off-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/299
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Ossama Othman <ossama.othman@intel.com>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>