Index: src/thread.cpp
===================================================================
--- src/thread.cpp
+++ src/thread.cpp
@@ -77,7 +77,10 @@
 unsigned
 thread::hardware_concurrency() _NOEXCEPT
 {
-#if defined(CTL_HW) && defined(HW_NCPU)
+#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+    // Defer to the external threading implementation
+    return __libcpp_thread_hw_concurrency();
+#elif defined(CTL_HW) && defined(HW_NCPU)
     unsigned n;
     int mib[2] = {CTL_HW, HW_NCPU};
     std::size_t s = sizeof(n);