Make compilation warnings as errors

[wangkuiyi]

Compilation warnings are annoying. PaddlePaddle used to suffer from many warnings of "comparing signed and unsigned integers". These warnings make it hard to go over the build log and find out errors. We tried hard to fix warnings; it was a hard work. Recently, we see warnings again. With the growing of contributors, the trend is clear.

• I propose to add the compiler flag -Werror, which considers warnings and errors, and
• we'd only want to apply this to PaddlePaddle's code, but not its dependents built using CMake.

Thanks to @gangliao who pointed the solution

1. Add -Werror to CMAKE_CXX_FLAGS and CMAKE_C_FLAGS. This would apply -Werror to PaddlePaddle.

2. Define PADDLE_DEPS_CXX_FLAGS and PADDLE_DEPS_C_FLAGS, which don't include -Werror.

3. Edit /cmake/external/*.cmake, add set the CMAKE_CXX_FLAGS and CMAKE_C_FLAGS variables of these dependents to be the value of PADDLE_DEPS_CXX_FLAGS and PADDLE_DEPS_C_FLAGS.
   For example:

   ExternalProject_Add(
    extern_glog
    ...
    CMAKE_ARGS      -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
                    -DCMAKE_C_COMPILER=${PADDLE_DEPS_C_COMPILER}
                    -DCMAKE_CXX_FLAGS=${PADDLE_DEPS_CXX_FLAGS}
                    -DCMAKE_C_FLAGS=${PADDLE_DEPS_C_FLAGS}