IOT-1583: Fixing libcoap W4 warnings.

IOT-1583: Fixing libcoap W4 warnings.

This commit makes sure libcoap doesn't generate any W4
warnings when building with for Windows.

In addition to that automatic libcoap updates are enabled
by default for Jenkins builds and optional for regular
builds.

Change-Id: I676a98bd41c2aeb28fee7d5ed1283145fd306655
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18545
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>