Refine nstime_update().

Add missing #include <time.h>.  The critical time facilities appear to
have been transitively included via unistd.h and sys/time.h, but in
principle this omission was capable of having caused
clock_gettime(CLOCK_MONOTONIC, ...) to have been overlooked in favor of
gettimeofday(), which in turn could cause spurious non-monotonic time
updates.

Refactor nstime_get() out of nstime_update() and add configure tests for
all variants.

Add CLOCK_MONOTONIC_RAW support (Linux-specific) and
mach_absolute_time() support (OS X-specific).

Do not fall back to clock_gettime(CLOCK_REALTIME, ...).  This was a
fragile Linux-specific workaround, which we're unlikely to use at all
now that clock_gettime(CLOCK_MONOTONIC_RAW, ...) is supported, and if we
have no choice besides non-monotonic clocks, gettimeofday() is only
incrementally worse.

include/jemalloc/internal/nstime.h

@@ -1,9 +1,6 @@
 /******************************************************************************/
 #ifdef JEMALLOC_H_TYPES
 
-#define	JEMALLOC_CLOCK_GETTIME defined(_POSIX_MONOTONIC_CLOCK) \
-    && _POSIX_MONOTONIC_CLOCK >= 0
-
 typedef struct nstime_s nstime_t;
 
 /* Maximum supported number of seconds (~584 years). */