Created by: Xreki

Current PR often make the build of mobile failed. When I build for Android using the current develop branch, I met the following error:

• CMake warning

-- Configuring done
CMake Error at paddle/testing/CMakeLists.txt:10 (add_dependencies):
  The dependency target "paddle_memory" of target "paddle_gtest_main" does
  not exist.

• Make error

[ 84%] Building CXX object paddle/testing/CMakeFiles/paddle_gtest_main.dir/paddle_gtest_main.cc.o
In file included from /home/work/liuyiqun/PaddlePaddle/Paddle/paddle/platform/place.h:19:0,
                 from /home/work/liuyiqun/PaddlePaddle/Paddle/paddle/memory/memory.h:17,
                 from /home/work/liuyiqun/PaddlePaddle/Paddle/paddle/testing/paddle_gtest_main.cc:18:
/home/work/liuyiqun/PaddlePaddle/Paddle/paddle/platform/variant.h:17:28: fatal error: boost/config.hpp: No such file or directory
 #include <boost/config.hpp>
                            ^
compilation terminated.
make[2]: *** [paddle/testing/CMakeFiles/paddle_gtest_main.dir/paddle_gtest_main.cc.o] Error 1
make[1]: *** [paddle/testing/CMakeFiles/paddle_gtest_main.dir/all] Error 2
make: *** [all] Error 2

The error is caused by PR https://github.com/PaddlePaddle/Paddle/pull/5476. In fact, there is no need to build the source file paddle_gtest_main.cc.

We need to add CI of build_android back to make sure cross-compiling always successful.