Fixed DLOPEN define warning.

Fixed DLOPEN define warning.

Unconditionally defining DLOPEN_POSIX for cpluff was causing redefinintion errors once
it started being passed in on the command-line. Added conditional guards to prevent the
warnings.

Change-Id: Ieb40909b77710fa46209465e4e924569059738c5
Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/451
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>