Explanation for comment

lib/CMakeLists.txt

@@ -18,7 +18,8 @@ add_subdirectory(Serialization)
 add_subdirectory(Frontend)
 add_subdirectory(FrontendTool)
 add_subdirectory(Tooling)
-add_subdirectory(DirectoryWatcher)
+# iOS: this relies on Mac-specific stuff like FSEventStreamRef
+# add_subdirectory(DirectoryWatcher)
 add_subdirectory(Index)
 if(CLANG_ENABLE_STATIC_ANALYZER)
   add_subdirectory(StaticAnalyzer)