* Problem: TIPC availability check is too strict

Solution: at build time only check if the API is available. In the tests
do a first check and a skip if the functionality is not available.

TIPC needs an in-tree but not loaded by default kernel module, tipc.ko
to be loaded, which requires root, so it is unlikely to be available on
any build system by default.
This will allow most distributions to ship with TIPC support built in,
and to avoid tests failure if the module is not there.

* Problem: no Travis tests for TIPC

Solution: mark one job with sudo: required and load the kernel module

* Problem: CMake fails when test returns 77 (skip)

Solution: set property to let it mark the test as skipped as intended