use CLOCK_MONOTONIC in the timer if it's available

Linux sets _POSIX_MONOTONIC_CLOCK to 0 meaning it *might* be available,
so a sysconf check is necessary at runtime with a fallback to the
mandatory CLOCK_REALTIME clock.

test/include/test/timer.h

@@ -1,10 +1,20 @@
 /* Simple timer, for use in benchmark reporting. */
 
+#include <unistd.h>
 #include <sys/time.h>
 
+#define JEMALLOC_CLOCK_GETTIME defined(_POSIX_MONOTONIC_CLOCK) \
+    && _POSIX_MONOTONIC_CLOCK >= 0
+
 typedef struct {
+#if JEMALLOC_CLOCK_GETTIME
+	struct timespec tv0;
+	struct timespec tv1;
+	int clock_id;
+#else
 	struct timeval tv0;
 	struct timeval tv1;
+#endif
 } timedelta_t;
 
 void	timer_start(timedelta_t *timer);