message: fix build dependency between onnx-operators.proto and
  onnx.proto
diff: @@ -91,7 +91,7 @@ endif()
  # function(RELATIVE_PROTOBUF_GENERATE_CPP SRCS HDRS ROOT_DIR)
  # from https://github.com/tensorflow/tensorflow/blob/d2c3b873c6f8ff999a2e4ee707a84ff00d9c15a5/tensorflow/contrib/cmake/tf_core_framework.cmake
  # to solve the problem that customized dir can't be specified when calling PROTOBUF_GENERATE_CPP.
  -function(RELATIVE_PROTOBUF_GENERATE_CPP SRCS HDRS ROOT_DIR)
  +function(RELATIVE_PROTOBUF_GENERATE_CPP NAME SRCS HDRS ROOT_DIR DEPEND)
  if(NOT ARGN)
  message(SEND_ERROR "Error: RELATIVE_PROTOBUF_GENERATE_CPP() called without any proto files")
  return()
  @@ -172,9 +172,11 @@ function(RELATIVE_PROTOBUF_GENERATE_CPP SRCS HDRS ROOT_DIR)
  "${OUTPUT_PB_HEADER}"
  COMMAND ${ONNX_PROTOC_EXECUTABLE}
  ARGS ${PROTOC_ARGS}
  - DEPENDS ${GENERATED_PROTO}
  + DEPENDS ${GENERATED_PROTO} ${DEPEND}
  COMMENT "Running C++ protocol buffer compiler on ${GENERATED_PROTO}"
  VERBATIM )
  + add_custom_target(${NAME}
  + DEPENDS ${OUTPUT_PB_SRC} ${OUTPUT_PB_HEADER})
  endforeach()
  set_source_files_properties(${${SRCS}} ${${HDRS}} PROPERTIES GENERATED TRUE)
  @@ -182,10 +184,14 @@ function(RELATIVE_PROTOBUF_GENERATE_CPP SRCS HDRS ROOT_DIR)
  set(${HDRS} ${${HDRS}} PARENT_SCOPE)
  endfunction()
  -RELATIVE_PROTOBUF_GENERATE_CPP(PROTO_SRCS PROTO_HDRS ${ONNX_ROOT}
  +RELATIVE_PROTOBUF_GENERATE_CPP(gen_onnx_proto PROTO_SRCS PROTO_HDRS ${ONNX_ROOT} ""
  onnx/onnx.in.proto
  +)
  +RELATIVE_PROTOBUF_GENERATE_CPP(gen_onnx_operators_proto PROTO_SRCS2 PROTO_HDRS2 ${ONNX_ROOT} gen_onnx_proto
  onnx/onnx-operators.in.proto
  )
  +list(APPEND PROTO_SRCS ${PROTO_SRCS2})
  +list(APPEND PROTO_HDRS ${PROTO_HDRS2})
  file(GLOB_RECURSE onnx_src
  "${ONNX_ROOT}/onnx/*.h"
  