diff --git a/include/__threading_support b/include/__threading_support
index f4d41f7a1..0064c7d59 100644
--- a/include/__threading_support
+++ b/include/__threading_support
@@ -27,12 +27,24 @@
   #endif
 #endif
 
+#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL) && \
+    !__libcpp_has_include(<__external_threading>)
+// If the <__external_threading> header is absent, build libc++ against a
+// pthread-oriented thread api but leave out its implementation. This setup
+// allows building+testing of an externally-threaded library variant (on any
+// platform that supports pthreads). Here, an 'externally-threaded' library
+// variant is one where the implementation of the libc++ thread api is provided
+// as a separate library.
+#define _LIBCPP_HAS_THREAD_API_EXTERNAL_PTHREAD
+#endif
+
 #if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL) && \
     __libcpp_has_include(<__external_threading>)
 #include <__external_threading>
 #else
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || \
+    defined(_LIBCPP_HAS_THREAD_API_EXTERNAL_PTHREAD)
 #include <pthread.h>
 #include <sched.h>
 #endif
@@ -45,7 +57,8 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || \
+    defined(_LIBCPP_HAS_THREAD_API_EXTERNAL_PTHREAD)
 // Mutex
 typedef pthread_mutex_t __libcpp_mutex_t;
 #define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
@@ -134,7 +147,8 @@ void *__libcpp_tls_get(__libcpp_tls_key __key);
 _LIBCPP_THREAD_ABI_VISIBILITY
 void __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || \
+    defined(_LIBCPP_BUILDING_THREAD_API_EXTERNAL_PTHREAD)
 
 int __libcpp_recursive_mutex_init(__libcpp_mutex_t *__m)
 {
diff --git a/test/support/external_threads.cpp b/test/support/external_threads.cpp
index 5f7106b22..68ba96ff8 100644
--- a/test/support/external_threads.cpp
+++ b/test/support/external_threads.cpp
@@ -6,5 +6,5 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-#define _LIBCPP_HAS_THREAD_API_PTHREAD
+#define _LIBCPP_BUILDING_THREAD_API_EXTERNAL_PTHREAD
 #include <__threading_support>