diff --git a/libcxx/test/libcxx/test/config.py b/libcxx/test/libcxx/test/config.py
@@ -510,6 +510,10 @@ def configure_link_flags_cxx_library(self):
                 self.cxx.link_flags += [abs_path]
             else:
                 self.cxx.link_flags += ['-lc++']
+        # This needs to come after -lc++ as we want its unresolved thread-api symbols
+        # to be picked up from this one.
+        if self.get_lit_bool('libcxx_external_thread_api', default=False):
+            self.cxx.link_flags += ['-lc++external_threads']
 
     def configure_link_flags_abi_library(self):
         cxx_abi = self.get_lit_conf('cxx_abi', 'libcxxabi')

diff --git a/libcxx/test/libcxx/thread/thread.condition/thread.condition.condvar/native_handle.pass.cpp b/libcxx/test/libcxx/thread/thread.condition/thread.condition.condvar/native_handle.pass.cpp
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <condition_variable>
 

diff --git a/...ex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/native_handle.pass.cpp b/...ex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/native_handle.pass.cpp
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <mutex>
 

diff --git a/...equirements/thread.mutex.requirements.mutex/thread.mutex.recursive/native_handle.pass.cpp b/...equirements/thread.mutex.requirements.mutex/thread.mutex.recursive/native_handle.pass.cpp
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <mutex>
 

diff --git a/...cxx/thread/thread.threads/thread.thread.class/thread.thread.member/native_handle.pass.cpp b/...cxx/thread/thread.threads/thread.thread.class/thread.thread.member/native_handle.pass.cpp
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <thread>
 

diff --git a/libcxx/test/libcxx/thread/thread.threads/thread.thread.class/types.pass.cpp b/libcxx/test/libcxx/thread/thread.threads/thread.thread.class/types.pass.cpp
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <thread>
 

diff --git a/libcxx/test/lit.site.cfg.in b/libcxx/test/lit.site.cfg.in
@@ -26,6 +26,7 @@ config.has_libatomic            = "@LIBCXX_HAS_ATOMIC_LIB@"
 config.use_libatomic            = "@LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB@"
 
 config.libcxxabi_shared         = "@LIBCXXABI_ENABLE_SHARED@"
+config.libcxx_external_thread_api  = "@LIBCXX_HAS_EXTERNAL_THREAD_API@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")
diff --git a/libcxx/test/support/external_threads.cpp b/libcxx/test/support/external_threads.cpp
@@ -0,0 +1,10 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+#define _LIBCPP_BUILDING_EXTERNAL_THREADS
+#include <__threading_support>