Created by: wangkuiyi

As reported by @reyoung , given that we have cmake/external/protobuf.cmake, build rules defined in cmake/generic.cmake, e.g., cc_library, should be able to have parameters like DEPS protobuf. But if we write such build rules, CMake would complain that target protobuf doesn't exist:

CMake Error at cmake/generic.cmake:177 (add_dependencies):
  The dependency target "protobuf" of target "example_proto" does not exist.
Call Stack (most recent call first):
  cmake/generic.cmake:200 (cc_library)
  paddle/framework/CMakeLists.txt:6 (proto_library)