Index: libcxx/src/chrono.cpp
===================================================================
--- libcxx/src/chrono.cpp
+++ libcxx/src/chrono.cpp
@@ -10,11 +10,12 @@
 #include "cerrno"        // errno
 #include "system_error"  // __throw_system_error
 #include <time.h>        // clock_gettime, CLOCK_MONOTONIC and CLOCK_REALTIME
+#include <unistd.h>
 #include "include/apple_availability.h"
 
-#if !defined(__APPLE__)
+#if _POSIX_TIMERS > 0
 #define _LIBCPP_USE_CLOCK_GETTIME
-#endif // __APPLE__
+#endif // _POSIX_TIMERS
 
 #if defined(_LIBCPP_WIN32API)
 #define WIN32_LEAN_AND_MEAN