add in OS X Monotonic clock as well as handling fall-back incase the OSX/Linux system doesn't have a monotonic clock.

Code curtesy of Thomas Habets ( https://github.com/ThomasHabets/monotonic_clock )

--HG--
extra : rebase_source : 767f4fbc2b7bda8d59c7b10a24467043770e94f1

src/timer/unix/SDL_systimer.c

@@ -34,85 +34,113 @@
    for __USE_POSIX199309
    Tommi Kyntola (tommi.kyntola@ray.fi) 27/09/2005
 */
+/* Reworked monotonic clock to not assume the current system has one
+   as not all linux kernels provide a monotonic clock (yeah recent ones
+   probably do)
+   Also added OS X Monotonic clock support
+   Based on work in https://github.com/ThomasHabets/monotonic_clock
+ */
 #if HAVE_NANOSLEEP || HAVE_CLOCK_GETTIME
 #include <time.h>
 #endif
+#ifdef __APPLE__
+#include <mach/mach_time.h>
+#endif
 
 /* The first ticks value of the application */
-#ifdef HAVE_CLOCK_GETTIME
+#if HAVE_CLOCK_GETTIME
-static struct timespec start;
+static struct timespec start_ts;
-#else
+#elif defined(__APPLE__)
-static struct timeval start;
+static uint64_t start_mach;
-#endif /* HAVE_CLOCK_GETTIME */
+mach_timebase_info_data_t mach_base_info;
-
+#endif
+static SDL_bool has_monotonic_time = SDL_FALSE;
+static struct timeval start_tv;
 
 void
 SDL_StartTicks(void)
 {
     /* Set first ticks value */
 #if HAVE_CLOCK_GETTIME
-    clock_gettime(CLOCK_MONOTONIC, &start);
+    if (clock_gettime(CLOCK_MONOTONIC, &start_ts) == 0) {
-#else
+        has_monotonic_time = SDL_TRUE;
-    gettimeofday(&start, NULL);
+    } else
+#elif defined(__APPLE__)
+    start_mach = mach_absolute_time();
+    kern_return_t ret = mach_timebase_info(&mach_base_info);
+    if (ret == 0) {
+        has_monotonic_time = SDL_TRUE;
+    } else
 #endif
+    {
+        gettimeofday(&start_tv, NULL);
+    }
 }
 
 Uint32
 SDL_GetTicks(void)
 {
+    Uint32 ticks;
+    if (has_monotonic_time) {
 #if HAVE_CLOCK_GETTIME
-    Uint32 ticks;
+        struct timespec now;
-    struct timespec now;
+        clock_gettime(CLOCK_MONOTONIC, &now);
-
+        ticks = 
-    clock_gettime(CLOCK_MONOTONIC, &now);
+            (now.tv_sec - start_ts.tv_sec) * 1000 + (now.tv_nsec -
-    ticks =
+                                                 start_ts.tv_nsec) / 1000000;
-        (now.tv_sec - start.tv_sec) * 1000 + (now.tv_nsec -
+#elif defined(__APPLE__)
-                                              start.tv_nsec) / 1000000;
+        uint64_t now = mach_absolute_time();
-    return (ticks);
+        ticks = (now - start_mach) * mach_base_info.numer / mach_base_info.denom / 1000000;
-#else
-    Uint32 ticks;
-    struct timeval now;
-
-    gettimeofday(&now, NULL);
-    ticks =
-        (now.tv_sec - start.tv_sec) * 1000 + (now.tv_usec -
-                                              start.tv_usec) / 1000;
-    return (ticks);
 #endif
+    } else {
+        struct timeval now;
+
+        gettimeofday(&now, NULL);
+        ticks =
+            (now.tv_sec - start_tv.tv_sec) * 1000 + (now.tv_usec -
+                                                  start_tv.tv_usec) / 1000;
+    }
+    return (ticks);
 }
 
 Uint64
 SDL_GetPerformanceCounter(void)
 {
+    Uint64 ticks;
+    if (has_monotonic_time) {
 #if HAVE_CLOCK_GETTIME
-    Uint64 ticks;
+        struct timespec now;
-    struct timespec now;
 
-    clock_gettime(CLOCK_MONOTONIC, &now);
+        clock_gettime(CLOCK_MONOTONIC, &now);
-    ticks = now.tv_sec;
+        ticks = now.tv_sec;
-    ticks *= 1000000000;
+        ticks *= 1000000000;
-    ticks += now.tv_nsec;
+        ticks += now.tv_nsec;
-    return (ticks);
+#elif defined(__APPLE__)
-#else
+        ticks = mach_absolute_time();
-    Uint64 ticks;
-    struct timeval now;
-
-    gettimeofday(&now, NULL);
-    ticks = now.tv_sec;
-    ticks *= 1000000;
-    ticks += now.tv_usec;
-    return (ticks);
 #endif
+    } else {
+        struct timeval now;
+
+        gettimeofday(&now, NULL);
+        ticks = now.tv_sec;
+        ticks *= 1000000;
+        ticks += now.tv_usec;
+    }
+    return (ticks);
 }
 
 Uint64
 SDL_GetPerformanceFrequency(void)
 {
+    if (has_monotonic_time) {
 #if HAVE_CLOCK_GETTIME
-    return 1000000000;
+        return 1000000000;
-#else
+#elif defined(__APPLE__)
-    return 1000000;
+        return mach_base_info.denom / mach_base_info.numer * 1000000;
 #endif
+    } else {
+        return 1000000;
+    }
 }
 
 void