diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index ea73cec050e..d56f7af583e 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -222,6 +222,8 @@ else() # if this is included from LLVM's CMake
   if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
     # This looks wrong, but libcxx's build actually wants the header dir to be
     # the root build dir, not the include directory.
+    set(LIBCXX_BINARY_DIR ${LLVM_BINARY_DIR})
+    set(LIBCXX_SOURCE_DIR ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR})
     set(LIBCXX_HEADER_DIR ${LLVM_BINARY_DIR})
     set(CXX_HEADER_TARGET runtime-libcxx-headers)
     add_subdirectory(${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include ${CXX_HEADER_TARGET})