Add back Darwin/Mach/Apple portability

Quite a few of the changes against the MalcolmRobb/dump1090 fork are
incompatible with OSX.  This patch adds back that cross platform support
back.

compat/clock_gettime/clock_gettime.h

@@ -0,0 +1,26 @@
+#ifdef __APPLE__
+
+#ifndef CLOCK_GETTIME_H
+#define CLOCK_GETTIME_H
+
+#include <mach/mach_time.h>
+
+#ifndef CLOCKID_T
+#define CLOCKID_T
+typedef enum
+{
+    CLOCK_REALTIME,
+    CLOCK_MONOTONIC,
+    CLOCK_PROCESS_CPUTIME_ID,
+    CLOCK_THREAD_CPUTIME_ID
+} clockid_t;
+#endif // CLOCKID_T
+
+struct timespec;
+
+static mach_timebase_info_data_t __clock_gettime_inf;
+
+int clock_gettime(clockid_t clk_id, struct timespec *tp);
+
+#endif // CLOCK_GETTIME_H
+#endif // __APPLE__