fix arrow.

cpp/CMakeLists.txt

diff --git a/cpp/CMakeLists.txt b/cpp/CMakeLists.txt
index 253341d..4cefbfd 100755
--- a/cpp/CMakeLists.txt
+++ b/cpp/CMakeLists.txt
@@ -186,7 +186,10 @@
     message("BUILD_TEST is OFF, skipping test directory")
 endif ()
 
-add_subdirectory(examples)
+option(BUILD_EXAMPLES "Build examples (requires Arrow/Parquet)" OFF)
+if (BUILD_EXAMPLES)
+    add_subdirectory(examples)
+endif()
 
 if (EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/experiment/CMakeLists.txt")
     add_subdirectory(experiment)