Index: libcxx/include/CMakeLists.txt
===================================================================
--- libcxx/include/CMakeLists.txt
+++ libcxx/include/CMakeLists.txt
@@ -2,6 +2,23 @@
   set(LIBCXX_SUPPORT_HEADER_PATTERN PATTERN "support" EXCLUDE)
 endif()
 
+if (LIBCXX_NEEDS_SITE_CONFIG)
+  # Generate a custom __config header. The new header is created
+  # by prepending __config_site to the current __config header.
+  add_custom_command(OUTPUT ${LIBCXX_BINARY_DIR}/__generated_config
+    COMMAND ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
+      ${LIBCXX_BINARY_DIR}/__config_site
+      ${LIBCXX_SOURCE_DIR}/include/__config
+      -o ${LIBCXX_BINARY_DIR}/__generated_config
+    DEPENDS ${LIBCXX_SOURCE_DIR}/include/__config
+            ${LIBCXX_BINARY_DIR}/__config_site
+  )
+  # Add a target that executes the generation commands.
+  add_custom_target(generate_config_header ALL
+    DEPENDS ${LIBCXX_BINARY_DIR}/__generated_config)
+  set(generated_config_deps generate_config_header)
+endif()
+
 set(LIBCXX_HEADER_PATTERN
   PATTERN "*"
   PATTERN "CMakeLists.txt" EXCLUDE
@@ -16,6 +33,15 @@
     FILES_MATCHING
     ${LIBCXX_HEADER_PATTERN}
     )
+
+  if (LIBCXX_NEEDS_SITE_CONFIG)
+    # Copy the generated header as __config into build directory.
+    add_custom_command(
+        TARGET generate_config_header POST_BUILD
+        COMMAND ${CMAKE_COMMAND} -E copy
+                ${LIBCXX_BINARY_DIR}/__generated_config
+                ${LLVM_BINARY_DIR}/include/c++/v1/__config)
+  endif()
 endif()
 
 if (LIBCXX_INSTALL_HEADERS)
@@ -28,20 +54,6 @@
   )
 
   if (LIBCXX_NEEDS_SITE_CONFIG)
-    # Generate and install a custom __config header. The new header is created
-    # by  prepending __config_site to the current __config header.
-    add_custom_command(OUTPUT ${LIBCXX_BINARY_DIR}/__generated_config
-      COMMAND ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
-        ${LIBCXX_BINARY_DIR}/__config_site
-        ${LIBCXX_SOURCE_DIR}/include/__config
-        -o ${LIBCXX_BINARY_DIR}/__generated_config
-      DEPENDS ${LIBCXX_SOURCE_DIR}/include/__config
-              ${LIBCXX_BINARY_DIR}/__config_site
-    )
-    # Add a target that executes the generation commands.
-    add_custom_target(generate_config_header ALL
-      DEPENDS ${LIBCXX_BINARY_DIR}/__generated_config)
-    set(generated_config_deps generate_config_header)
     # Install the generated header as __config.
     install(FILES ${LIBCXX_BINARY_DIR}/__generated_config
       DESTINATION ${LIBCXX_INSTALL_PREFIX}include/c++/v1