Rename WITH_C_API into PADDLE_BUILD_INFERENCE_LIB

[wangkuiyi]

The term C_API has been pretty confusing. By reading our source code, I think it means the C-API for inference only, but not those for training. Let us use an accurate name for it.

[luotao1]

As WITH_C_API is used for a long time in old PaddlePaddle, users are familiar with it, we may remain it. And for fluid, we will add WITH_FLUID option for both training and inference.