paddle compilation speed up

1. Disable unnecessary things in cmake, for example:

cmake .. -DCMAKE_BUILD_TYPE=RelWithDebInfo -DWITH_DOC=OFF -DWITH_GPU=OFF -DWITH_DISTRIBUTE=OFF -DWITH_MKL=ON -DWITH_GOLANG=OFF -DWITH_SWIG_PY=OFF -DWITH_STYLE_CHECK=OFF -DWITH_TESTING=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON

List of all flags: http://www.paddlepaddle.org/docs/develop/documentation/en/getstarted/build_and_install/build_from_source_en.html#bool-type-options

2. Use ccache: https://askubuntu.com/questions/470545/how-do-i-set-up-ccache
3. For changes in core.so, use target copy_paddle_pybind: make copy_paddle_pybind
4. Don't use just -j without providing core number for cmake unless you have enormous amount of RAM on your machine (64GB is not enough).