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/jemalloc_internal_decls.h

@@ -19,6 +19,10 @@
 #  include <pthread.h>
 #  include <errno.h>
 #  include <sys/time.h>
+#  include <time.h>
+#  ifdef JEMALLOC_HAVE_MACH_ABSOLUTE_TIME
+#    include <mach/mach_time.h>
+#  endif
 #endif
 #include <sys/types.h>