add back third_party/protobuf, but it won't be used in normal builds.

Pinned protobuf to v3.1.0

Removed the USE_SYSTEM_PROTOBUF option in cmake. It is no longer used.

.gitmodules

@@ -22,3 +22,6 @@
 [submodule "third_party/benchmark"]
 	path = third_party/benchmark
 	url = https://github.com/google/benchmark.git
+[submodule "third_party/protobuf"]
+	path = third_party/protobuf
+	url = https://github.com/google/protobuf.git

CMakeLists.txt

@@ -44,7 +44,6 @@ option(USE_ZMQDB "Use ZMQ" OFF)
 option(USE_ROCKSDB "Use RocksDB" OFF)
 option(USE_MPI "Use MPI" ON)
 option(BUILD_SHARED_LIBS "Build libcaffe2.so" ON)
-option(USE_SYSTEM_PROTOBUF "Use system-provided protobuf" ON)
 option(USE_OPENMP "Use OpenMP for parallel code" ON)
 option(BUILD_PYTHON "Build python binaries" ON)
 

caffe/proto/CMakeLists.txt

@@ -8,8 +8,4 @@ add_library(Caffe_PROTO SHARED ${Caffe_PROTO_HEADERS} ${Caffe_PROTO_SRCS})
 target_include_directories(Caffe_PROTO PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
 install(TARGETS Caffe_PROTO DESTINATION ${CMAKE_INSTALL_PREFIX}/lib)
 
-if(USE_SYSTEM_PROTOBUF)
-  target_link_libraries(Caffe_PROTO protobuf)
-else()
-  add_dependencies(Caffe_PROTO protobuf_external)
-endif()
+target_link_libraries(Caffe_PROTO protobuf)

caffe2/proto/CMakeLists.txt

@@ -8,8 +8,4 @@ add_library(Caffe2_PROTO SHARED ${Caffe2_PROTO_HEADERS} ${Caffe2_PROTO_SRCS})
 target_include_directories(Caffe2_PROTO PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
 install(TARGETS Caffe2_PROTO DESTINATION ${CMAKE_INSTALL_PREFIX}/lib)
 
-if (USE_SYSTEM_PROTOBUF)
-  target_link_libraries(Caffe2_PROTO protobuf)
-else()
-  add_dependencies(Caffe2_PROTO protobuf_external)
-endif()
+target_link_libraries(Caffe2_PROTO protobuf)