Need a better build script for CMake

[apeforest]

Description

The current instruction for users to build from source using Cmake is not very user-friendly. Users have to remember each flag whenever they build mxnet.

It will be nice to have a simple setup.py and users only need to run the following as Pytorch:

python setup.py install

[leezu]

We also need to handle other language bindings. But yes, the user experience can be improved.

[apeforest]

Closing the issue as the current cmake build step has been greatly simplified using a config.cmake file thanks to @leezu