Add Hurd support

• configure.ac: Define QB_GNU. Add a check for a working clock_getres for the CLOCK_MONOTONIC option defining HAVE_CLOCK_GETRES_MONOTONIC.
• lib/log_thread.c: Replace second argument of qb_log_thread_priority_set(): logt_sched_param.sched_priority by 0 when not supported by the OS.
• lib/util.c: Use the CLOCK_REALTIME option in clock_getres() if HAVE_CLOCK_GETRES_MONOTONIC os not defined.