diff --git a/src/impl/unix.cc b/src/impl/unix.cc
index 6c1dbe4..3cfb077 100755
--- a/src/impl/unix.cc
+++ b/src/impl/unix.cc
@@ -27,6 +27,7 @@
 #include <sys/time.h>
 #include <time.h>
 #ifdef __MACH__
+#include <AvailabilityMacros.h>
 #include <mach/clock.h>
 #include <mach/mach.h>
 #endif