Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -202,6 +202,7 @@
    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)
+option(LIBCXX_HAS_WIN32_THREADS "Ignore auto-detection and force use of win32 threads" OFF)
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
   "Build libc++ with an externalized threading API.
    This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON." OFF)
@@ -273,6 +274,10 @@
     message(FATAL_ERROR "LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY can only be set "
                         "to ON when LIBCXX_ENABLE_THREADS is also set to ON.")
   endif()
+  if(LIBCXX_HAS_WIN32_THREADS)
+    message(FATAL_ERROR "LIBCXX_HAS_WIN32_THREADS can only be set to ON"
+                        " when LIBCXX_ENABLE_THREADS is also set to ON.")
+  endif()
 
 endif()
 
@@ -287,6 +292,19 @@
                         "and LIBCXX_HAS_PTHREAD_API cannot be both"
                         "set to ON at the same time.")
   endif()
+  if (LIBCXX_HAS_WIN32_THREADS)
+    message(FATAL_ERROR "The options LIBCXX_HAS_EXTERNAL_THREAD_API"
+                        "and LIBCXX_HAS_WIN32_THREADS cannot be both"
+                        "set to ON at the same time.")
+  endif()
+endif()
+
+if (LIBCXX_HAS_PTHREAD_API)
+  if (LIBCXX_HAS_WIN32_THREADS)
+    message(FATAL_ERROR "The options LIBCXX_HAS_PTHREAD_API"
+                        "and LIBCXX_HAS_WIN32_THREADS cannot be both"
+                        "set to ON at the same time.")
+  endif()
 endif()
 
 # Ensure LLVM_USE_SANITIZER is not specified when LIBCXX_GENERATE_COVERAGE
@@ -613,6 +631,7 @@
 
 config_define_if(LIBCXX_HAS_PTHREAD_API _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define_if(LIBCXX_HAS_EXTERNAL_THREAD_API _LIBCPP_HAS_THREAD_API_EXTERNAL)
+config_define_if(LIBCXX_HAS_WIN32_THREADS _LIBCPP_HAS_THREAD_API_WIN32)
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
Index: include/__config_site.in
===================================================================
--- include/__config_site.in
+++ include/__config_site.in
@@ -23,6 +23,7 @@
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
+#cmakedefine _LIBCPP_HAS_THREAD_API_WIN32
 #cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_NO_VCRUNTIME