remove use of jack_get_time() from ARDOUR::get_microseconds(), use clock_gettime() instead since this function is never used in reference to a backend clock

libs/ardour/ardour/ardour.h

@@ -67,9 +67,7 @@ namespace ARDOUR {
 	bool translations_are_enabled ();
 	bool set_translations_enabled (bool);
 
-	static inline microseconds_t get_microseconds () {
-		return (microseconds_t) jack_get_time();
-	}
+	microseconds_t get_microseconds ();
 
 	void setup_fpu ();
 	std::vector<SyncSource> get_available_sync_options();

libs/ardour/globals.cc

@@ -29,6 +29,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <time.h>
 
 #ifdef WINDOWS_VST_SUPPORT
 #include <fst.h>
@@ -535,3 +536,18 @@ ARDOUR::get_available_sync_options ()
 
 	return ret;
 }
+
+/** Return a monotonic value for the number of microseconds that have elapsed
+ * since an arbitrary zero origin.
+ */
+ 
+microseconds_t
+ARDOUR::get_microseconds ()
+{
+	struct timespec ts;
+	if (clock_gettime (CLOCK_MONOTONIC, &ts) != 0) {
+		/* EEEK! */
+		return 0;
+	}
+	return (microseconds_t) ts.tv_sec * 1000000 + (ts.tv_nsec/1000);
+}