Fix the problem that protobuf cannot be used as a DEPS argument in cc…

[Xreki]

Fix #2568
使用了#2569 中的测试代码paddle/framework/example.proto 和 paddle/framework/example.cc，
paddle/framework/CMakelist.txt中增加如下内容：

set(CUR_PROTO_GEN
      ${CMAKE_CURRENT_BINARY_DIR}/example.pb.h
      ${CMAKE_CURRENT_BINARY_DIR}/example.pb.cc)
set(PROTO_GEN
      ${PROTO_GEN}
      ${CUR_PROTO_GEN})
add_custom_command(OUTPUT ${CUR_PROTO_GEN}
      COMMAND ${PROTOBUF_PROTOC_EXECUTABLE} 
      --cpp_out ${CMAKE_CURRENT_BINARY_DIR}
      --proto_path ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/example.proto
      DEPENDS protobuf)

#protobuf_generate_cpp(example_proto_src example_proto_header example.proto)
cc_library(example_proto SRCS ${CMAKE_CURRENT_BINARY_DIR}/example.pb.cc)
include_directories(${CMAKE_BINARY_DIR})
cc_test(example_test SRCS example_test.cc DEPS example_proto protobuf)

注意，生成"*.pb.cc"和"*.pb.h"采用paddle/proto/CMakelist.txt中的方式, 是因为CMake系统自带的macro protobuf_generate_cpp存在以下问题：

• Paddle在交叉编译和设置了PROTOBUF_ROOT时不会调用find_package(Protobuf)，会出现如下错误：

CMake Error at paddle/framework/CMakeLists.txt:18 (protobuf_generate_cpp):
  Unknown CMake command "protobuf_generate_cpp".

• protobuf_generate_cpp不能设置依赖关系，如果需要使用third_party目录下编译好的protoc，使用make -j12编译可能会出现出现错误：

make[2]: *** No rule to make target `../third_party/install/protobuf/bin/protoc', needed by `paddle/framework/example.pb.cc'.  Stop.
make[1]: *** [paddle/framework/CMakeFiles/example_proto.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

[reyoung]

有一点问题不确定。。如果没问题，就merge吧

[reyoung]

不太清楚STATIC_LIBRARY_SUFFIX定义在哪？

https://cmake.org/cmake/help/v3.0/variable/CMAKE_STATIC_LIBRARY_SUFFIX.html
是这个变量么？

[Xreki]

哦，这个是我定义在system.cmake里面的。cmake系统本身有定义这个变量吗，我看下

[Xreki]

Done, 都替换成CMake系统变量了，诸如：

• CMAKE_STATIC_LIBRARY_PREFIX
• CMAKE_STATIC_LIBRARY_SUFFIX
• ...

[gangliao]

LGTM