New macro (JCGO_MONOTWAIT) to use monotonic clock in pthread timed-wait

* include/jcgothrd.h: Include time.h if JCGO_MONOTWAIT (only if not
JCGO_OS2, not JCGO_SOLTHR, and not JCGO_WIN32).
* include/jcgothrd.h (JCGO_MONOTWAIT): Recognize new macro (only if
not JCGO_OS2, not JCGO_SOLTHR, and not JCGO_WIN32).
* include/jcgothrd.h (JCGO_EVENT_CONDTMWAIT): New macro (defined
depending on JCGO_MONOTWAIT).
* include/jcgothrd.h (JCGO_EVENTTIME_T, JCGO_EVENTTIME_PREPARE): Use
clock_gettime (instead of gettimeofday) if JCGO_MONOTWAIT.
* include/jcgothrd.h (JCGO_EVENT_TIMEDWAIT): Use JCGO_EVENT_CONDTMWAIT
instead of pthread_cond_timedwait (only if not JCGO_OS2, not
JCGO_SOLTHR, and not JCGO_WIN32).
README, Samples: Add information about JCGO_MONOTWAIT macro