Created by: wangkuiyi

For each target, CMake requires a call to target_link_libraries to specify the to-be-linked libraries and a call to add_dependencies to specify the dependencies. An example is https://github.com/PaddlePaddle/Paddle/pull/2326/files#diff-7a3600c73304487a6d5f6bd3fa3fd662R41

It seems that we need a way to associate each 3rd-party dependency to its libraries.

Currently, we specify a 3rd-party dependency, for example, glog, by calling ExternalProject_Add in cmake/external/*.cmake. For example, https://github.com/PaddlePaddle/Paddle/blob/develop/cmake/external/glog.cmake#L30. And we specify the libraries of this 3rd-party dependency by defining a variable, like GLOG_LIBRARIES in https://github.com/PaddlePaddle/Paddle/blob/develop/cmake/external/glog.cmake#L24.

Could we rename GLOG_LIBRARIES to be glog_libs in cmake/external/glog.cmake, and use the following directive to specify 3rd-party dependencies,

cc_library(some_target SRCS some_source.cc DEPS some_paddle_deps 3RDS glog)

where the definition of cc_library calls somethingl like ${cc_library_DEP}_libs to refer to glog_libs defined in cmake/external/glog.cmake?