Conditionally compile extras like benchmarks and demos (#4094)

Summary:
While embedding faiss as a subproject, these targets are quite useless. This PR adss a new `FAISS_ENABLE_EXTRAS` CMake option to exclude them, default is ON to preserve current behaviour.

Pull Request resolved: #4094

Reviewed By: satymish

Differential Revision: D67345666

Pulled By: gtwang01

fbshipit-source-id: c8d9ef14b21816e45f681412bda257192f5365d0

Author: artem

CMakeLists.txt

@@ -64,6 +64,7 @@ option(FAISS_ENABLE_CUVS "Enable cuVS for GPU indexes." OFF)
 option(FAISS_ENABLE_ROCM "Enable ROCm for GPU indexes." OFF)
 option(FAISS_ENABLE_PYTHON "Build Python extension." ON)
 option(FAISS_ENABLE_C_API "Build C API." OFF)
+option(FAISS_ENABLE_EXTRAS "Build extras like benchmarks and demos" ON)
 option(FAISS_USE_LTO "Enable Link-Time optimization" OFF)
 
 if(FAISS_ENABLE_GPU)
@@ -103,10 +104,11 @@ if(FAISS_ENABLE_C_API)
   add_subdirectory(c_api)
 endif()
 
-add_subdirectory(demos)
-add_subdirectory(benchs)
-add_subdirectory(tutorial/cpp)
-
+if(FAISS_ENABLE_EXTRAS)
+  add_subdirectory(demos)
+  add_subdirectory(benchs)
+  add_subdirectory(tutorial/cpp)
+endif()
 
 # CTest must be included in the top level to enable `make test` target.
 include(CTest)