Build: add gtest libs automatically

Build: add gtest libs automatically

Once gtest's sconfile is imported, build flags are propagated.

Deeper cleanup and effort to make tests less linux specific
 by avoiding to add gtest's pthread dependencies
 in sub scons files.

Bug: https://jira.iotivity.org/browse/IOT-863
Bug-AGL: https://jira.automotivelinux.org/browse/SPEC-158
Change-Id: I4ddbf3e1eef8648ceaeae6c62175308cb6f3fec1
Origin: https://github.com/tizenteam/iotivity/
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/249
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: David Antler <david.a.antler@intel.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Jon A. Cruz <jon@joncruz.org>