endian: MacOS X support for endian conversion macros

betohX/htobeX are only available on GNU/Linux.  This change introduces a
header file that maps these functions to the ones available on MacOS X.
diff --git a/lib/receiver/receiver_impl.cc b/lib/receiver/receiver_impl.cc
index a31721d..25dc994 100644
--- a/lib/receiver/receiver_impl.cc
+++ b/lib/receiver/receiver_impl.cc
@@ -38,6 +38,7 @@
 #include <iomanip>
 #include <assert.h>
 #include <boost/scoped_ptr.hpp>
+#include <gsm/endian.h>
 
 //files included for debuging
 //#include "plotting/plotting.hpp"