Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -126,6 +126,7 @@
   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
+option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 
 # Misc options ----------------------------------------------------------------
 # FIXME: Turn -pedantic back ON. It is currently off because it warns
@@ -172,6 +173,11 @@
                       " when LIBCXX_ENABLE_THREADS is also set to OFF.")
 endif()
 
+if(LIBCXX_HAS_PTHREAD_API AND NOT LIBCXX_ENABLE_THREADS)
+  message(FATAL_ERROR "LIBCXX_HAS_PTHREAD_API can only be set to ON"
+                      " when LIBCXX_ENABLE_THREADS is also set to ON.")
+endif()
+
 # Ensure LLVM_USE_SANITIZER is not specified when LIBCXX_GENERATE_COVERAGE
 # is ON.
 if (LLVM_USE_SANITIZER AND LIBCXX_GENERATE_COVERAGE)
@@ -384,6 +390,7 @@
 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
@@ -813,19 +813,23 @@
 #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
 
+#if defined(_LIBCPP_HAS_NO_THREADS) && defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#  error _LIBCPP_HAS_THREAD_API_PTHREAD may only be defined when \
+         _LIBCPP_HAS_NO_THREADS is not defined.
+#endif
 
 #if defined(_LIBCPP_HAS_NO_MONOTONIC_CLOCK) && !defined(_LIBCPP_HAS_NO_THREADS)
 #  error _LIBCPP_HAS_NO_MONOTONIC_CLOCK may only be defined when \
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