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.c

@@ -0,0 +1,49 @@
+/*
+ * clock_nanosleep.c - clock_nanosleep() replacement
+ */
+
+#ifdef __APPLE__
+
+#include "clock_nanosleep.h"
+#include <errno.h>                           // for errno, EINVAL
+#include <time.h>                            // for nanosleep, NULL
+#include "../clock_gettime/clock_gettime.h"  // for clock_gettime
+
+int clock_nanosleep(clockid_t id, int flags, const struct timespec *ts,
+                    struct timespec *ots) {
+    int ret;
+
+    if (id != CLOCK_REALTIME)
+        return EINVAL;
+
+    if (flags & TIMER_ABSTIME) {
+        struct timespec mine;
+
+        if (clock_gettime(id, &mine))
+            return errno;
+
+        if (mine.tv_sec > ts->tv_sec)
+            return 0; // behind schedule
+
+        if (mine.tv_nsec > ts->tv_nsec) {
+            if (mine.tv_sec == ts->tv_sec)
+                return 0; // behind schedule too
+
+            mine.tv_nsec = 1000000000 + ts->tv_nsec - mine.tv_nsec;
+            mine.tv_sec++;
+        }
+        else
+            mine.tv_nsec = ts->tv_nsec - mine.tv_nsec;
+
+        mine.tv_sec = ts->tv_sec - mine.tv_sec;
+
+        /* With TIMER_ABSTIME, clock_nanosleep ignores <ots> */
+        ret = nanosleep(&mine, NULL);
+    }
+    else
+        ret = nanosleep(ts, ots);
+
+    return ret ? errno : 0;
+}
+
+#endif // __APPLE__