Index: clang-tools-extra/trunk/CMakeLists.txt
===================================================================
--- clang-tools-extra/trunk/CMakeLists.txt
+++ clang-tools-extra/trunk/CMakeLists.txt
@@ -1,6 +1,13 @@
-option(CLANGD_BUILD_XPC "Build XPC Support For Clangd." OFF)
-if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-  set(CLANGD_BUILD_XPC ON CACHE BOOL "" FORCE)
+if (NOT DEFINED CLANGD_BUILD_XPC)
+  if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+    set(CLANGD_BUILD_XPC_DEFAULT ON)
+  else ()
+    set(CLANGD_BUILD_XPC_DEFAULT OFF)
+  endif ()
+
+  set(CLANGD_BUILD_XPC ${CLANGD_BUILD_XPC_DEFAULT} CACHE BOOL "Build XPC Support For Clangd." FORCE)
+
+  unset(CLANGD_BUILD_XPC_DEFAULT)
 endif ()
 
 add_subdirectory(clang-apply-replacements)