Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -128,18 +128,22 @@
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()
 foreach(proj ${LLVM_ENABLE_PROJECTS})
-  set(PROJ_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  string(TOUPPER "${proj}" upper_proj)
+  STRING(REGEX REPLACE "-" "_" upper_proj ${upper_proj})
+  if(NOT LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR)
+    set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR   "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  endif()
+  set(PROJ_DIR "${LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR}")
   if(NOT EXISTS "${PROJ_DIR}" OR NOT IS_DIRECTORY "${PROJ_DIR}")
     message(FATAL_ERROR "LLVM_ENABLE_PROJECTS requests ${proj} but directory not found: ${PROJ_DIR}")
   endif()
-  string(TOUPPER "${proj}" upper_proj)
-  STRING(REGEX REPLACE "-" "_" upper_proj ${upper_proj})
-  set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR   "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
   # There is a widely spread opinion that clang-tools-extra should be merged
   # into clang. The following simulates it by always enabling clang-tools-extra
   # when enabling clang.
   if (proj STREQUAL "clang")
-    set(LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../clang-tools-extra")
+    if(NOT LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR)
+      set(LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../clang-tools-extra")
+    endif()
   endif()
 endforeach()