Index: llvm/cmake/modules/CrossCompile.cmake
===================================================================
--- llvm/cmake/modules/CrossCompile.cmake
+++ llvm/cmake/modules/CrossCompile.cmake
@@ -41,10 +41,15 @@
     if (buildtype)
       set(build_type_flags "-DCMAKE_BUILD_TYPE=${buildtype}")
     endif()
+	if (LLVM_EXTERNAL_CLANG_SOURCE_DIR)
+	  # Propagate LLVM_EXTERNAL_CLANG_SOURCE_DIR so that clang-tblgen can be built
+	  set(external_clang_dir "-DLLVM_EXTERNAL_CLANG_SOURCE_DIR=${LLVM_EXTERNAL_CLANG_SOURCE_DIR}")
+	endif()
     execute_process(COMMAND ${CMAKE_COMMAND} ${build_type_flags}
         -G "${CMAKE_GENERATOR}" -DLLVM_TARGETS_TO_BUILD=${LLVM_TARGETS_TO_BUILD}
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
+        ${external_clang_dir}
       WORKING_DIRECTORY ${LLVM_${target_name}_BUILD} )
   endif(NOT IS_DIRECTORY ${LLVM_${target_name}_BUILD})