Use #if for HAVE_CLOCK_GETTIME everywhere

ifdef is useless, since the value is always defined (either to 1 or 0) in config.h.

Fix inspired by

  https://svnweb.freebsd.org/ports/head/audio/jack/files/patch-drivers_dummy_dummy__driver.c?view=markup

Also simplified #if condition in libjack/time.c - it's easier to read a positive than a negated
condition.

jackd/engine.c

@@ -627,7 +627,7 @@ jack_process_internal (jack_engine_t *engine, JSList *node,
    to know that that jack_get_microseconds() is monotonic.
  */
 
-#ifdef HAVE_CLOCK_GETTIME
+#if HAVE_CLOCK_GETTIME
 static const int system_clock_monotonic = 1;
 #else
 static const int system_clock_monotonic = 0;

libjack/time.c

@@ -48,7 +48,7 @@ jack_clock_source_name (jack_timer_type_t src)
 	case JACK_TIMER_HPET:
 		return "hpet";
 	case JACK_TIMER_SYSTEM_CLOCK:
-#ifdef HAVE_CLOCK_GETTIME
+#if HAVE_CLOCK_GETTIME
 		return "system clock via clock_gettime";
 #else
 		return "system clock via gettimeofday";
@@ -60,16 +60,17 @@ jack_clock_source_name (jack_timer_type_t src)
 	return "unknown";
 }
 
-#ifndef HAVE_CLOCK_GETTIME
+#if HAVE_CLOCK_GETTIME
 
 jack_time_t
 jack_get_microseconds_from_system (void)
 {
 	jack_time_t jackTime;
-	struct timeval tv;
+	struct timespec time;
 
-	gettimeofday (&tv, NULL);
-	jackTime = (jack_time_t)tv.tv_sec * 1000000 + (jack_time_t)tv.tv_usec;
+	clock_gettime (CLOCK_MONOTONIC, &time);
+	jackTime = (jack_time_t)time.tv_sec * 1e6 +
+		   (jack_time_t)time.tv_nsec / 1e3;
 	return jackTime;
 }
 
@@ -79,11 +80,10 @@ jack_time_t
 jack_get_microseconds_from_system (void)
 {
 	jack_time_t jackTime;
-	struct timespec time;
+	struct timeval tv;
 
-	clock_gettime (CLOCK_MONOTONIC, &time);
-	jackTime = (jack_time_t)time.tv_sec * 1e6 +
-		   (jack_time_t)time.tv_nsec / 1e3;
+	gettimeofday (&tv, NULL);
+	jackTime = (jack_time_t)tv.tv_sec * 1000000 + (jack_time_t)tv.tv_usec;
 	return jackTime;
 }