diff --git a/cmake/modules/AddLLVM.cmake b/cmake/modules/AddLLVM.cmake
index ebd17a0cf85..6c21a46b97d 100644
--- a/cmake/modules/AddLLVM.cmake
+++ b/cmake/modules/AddLLVM.cmake
@@ -694,6 +694,10 @@ macro(add_llvm_external_project name)
     set(add_llvm_external_dir ${name})
   endif()
   canonicalize_tool_name(${name} nameUPPER)
+  if(NOT DEFINED LLVM_TOOL_${projectName}_BUILD)
+    option(LLVM_TOOL_${name}_BUILD
+           "Whether to build ${name} as part of LLVM" On)
+  endif()
   if (LLVM_TOOL_${nameUPPER}_BUILD)
     if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${add_llvm_external_dir}/CMakeLists.txt)
         add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/${add_llvm_external_dir} ${add_llvm_external_dir})