threads_posix: Use monotonic clock for condition variables where possible

The pthread_condattr_setclock() function allows one to specify that the
condition variable use CLOCK_MONOTONIC as the base for timed waits. This
is desirable to avoid premature timeouts if the system time is adjusted
while a timed wait is occurring. Add a check for the availability of
this function and use it where possible.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>