diff --git a/thpool.c b/thpool.c
index d320f8f..329f3c6 100644
--- a/thpool.c
+++ b/thpool.c
@@ -8,7 +8,13 @@
  *
  ********************************/
 
+#if defined(__APPLE__)
+#include <AvailabilityMacros.h>
+#else
+#ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L
+#endif
+#endif
 #include <unistd.h>
 #include <signal.h>
 #include <stdio.h>