build: fix compilation on Alpine 3.5

The cbus hang test uses glibc pthread mutex implementation details.
The reason why mutex implementation details is used:
"For the bug reproducing the canceled thread must be canceled
during processing cpipe_flush_cb. We need to synchronize
the main thread and the canceled worker thread for that.
So, thread synchronization has been realized by means of
endpoint's mutex internal field(__data.__lock)."
Therefore, it should not compile in case of using another library.