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/README

@@ -0,0 +1,31 @@
+/*
+ *  clock_gettime_stub.c
+ *  gcc -Wall -c clock_gettime_stub.c
+ *  posix realtime functions; MacOS user space glue
+ */
+
+/*  @comment
+ *  other possible implementation using intel builtin rdtsc
+ *  rdtsc-workaround: http://www.mcs.anl.gov/~kazutomo/rdtsc.html
+ *
+ *  we could get the ticks by doing this
+ *
+ *  __asm __volatile("mov %%ebx, %%esi\n\t"
+ *  		"cpuid\n\t"
+ *  		"xchg %%esi, %%ebx\n\t"
+ *  		"rdtsc"
+ *  		: "=a" (a),
+ *  		  "=d" (d)
+ * 	);
+
+ *  we could even replace our tricky sched_yield call by assembly code to get a better accurency,
+ *  anyway the following C stub will satisfy 99% of apps using posix clock_gettime call,
+ *  moreover, the setter version (clock_settime) could be easly written using mach primitives:
+ *  http://www.opensource.apple.com/source/xnu/xnu-${VERSION}/osfmk/man/ (clock_[set|get]_time)
+ *
+ *  hackers don't be crackers, don't you use a flush toilet?
+ *
+ *
+ *  @see draft: ./posix-realtime-stub/posix-realtime-stub.c
+ *
+ */