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_nanosleep/clock_nanosleep.h

@@ -0,0 +1,27 @@
+#ifdef __APPLE__
+
+#ifndef CLOCK_NANOSLEEP_H
+#define CLOCK_NANOSLEEP_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
+
+#ifndef TIMER_ABSTIME
+#define TIMER_ABSTIME 1
+#endif // TIMER_ABSTIME
+
+struct timespec;
+
+int clock_nanosleep (clockid_t id, int flags, const struct timespec *ts,
+                     struct timespec *ots);
+
+#endif //CLOCK_NANOSLEEP_H
+#endif // __APPLE__