More MinGW64 related cleanups.

runtime/mercury_mm_own_stacks.c:
    Use Sleep() instead of sleep() on Windows.

    Avoid warnings from the C compiler.

samples/concurrency/dining_philosophers/philo.m:
samples/concurrency/dining_philosophers/philo3.m:
	Use '.' as a module qualifier in a spot.

	Replace calls to obsolete predicates.