OS X compatibility cleanups.

Reattach the licenses for the compat stuff to the source.
Only build/link the compat stuff when it's needed.
Rename compat/util.h so it's not confused with util.h.
Move all the platform specific defines inside compat/compat.h.

dump1090.h

@@ -82,6 +82,8 @@
     #include "winstubs.h" //Put everything Windows specific in here
 #endif
 
+#include "compat/compat.h"
+
 // Avoid a dependency on rtl-sdr except where it's really needed.
 typedef struct rtlsdr_dev rtlsdr_dev_t;