Linking against version of libcoap in favor of the version in extlibs.

Linking against version of libcoap in favor of the version in extlibs.

Bug: https://jira.iotivity.org/browse/IOT-2055
Change-Id: I3ca9b87ab33aeeb18607c582f2bdccfaa18b3d2f
Signed-off-by: Ziran Sun <ziran.sun@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/19063
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>