Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -384,6 +384,7 @@
 config_define_if_not(LIBCXX_ENABLE_MONOTONIC_CLOCK _LIBCPP_HAS_NO_MONOTONIC_CLOCK)
 config_define_if_not(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS)
 
+config_define_if(LIBCXX_HAS_PTHREAD_API _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
 if (LIBCXX_NEEDS_SITE_CONFIG)
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -813,17 +813,17 @@
 #endif
 
 // Thread API
-#ifndef _LIBCPP_HAS_NO_THREADS
+#if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 # if defined(__FreeBSD__) || \
     defined(__NetBSD__) || \
     defined(__linux__) || \
     defined(__APPLE__) || \
     defined(__CloudABI__) || \
     defined(__sun__)
-#  define _LIBCPP_THREAD_API_PTHREAD
+#  define _LIBCPP_HAS_THREAD_API_PTHREAD
 # else
 #  error "No thread API"
-# endif // _LIBCPP_THREAD_API
+# endif // _LIBCPP_HAS_THREAD_API
 #endif // _LIBCPP_HAS_NO_THREADS
 
 
Index: include/__config_site.in
===================================================================
--- include/__config_site.in
+++ include/__config_site.in
@@ -19,5 +19,6 @@
 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
+#cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 
 #endif // _LIBCPP_CONFIG_SITE
Index: include/__threading_support
===================================================================
--- include/__threading_support
+++ include/__threading_support
@@ -19,14 +19,14 @@
 
 #ifndef _LIBCPP_HAS_NO_THREADS
 
-#if defined(_LIBCPP_THREAD_API_PTHREAD)
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 #include <pthread.h>
 #include <sched.h>
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(_LIBCPP_THREAD_API_PTHREAD)
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 
 // Mutex
 #define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
@@ -194,7 +194,7 @@
     pthread_setspecific(__key, __p);
 }
 
-#else // !_LIBCPP_THREAD_API_PTHREAD
+#else // !_LIBCPP_HAS_THREAD_API_PTHREAD
   #error "No thread API selected."
 #endif