Adding cmake build to cpp splines

[aswinpajayan]

Hi, This project helped me a lot during my academic projects. I had to spend a good amount of time figuring out how to build this project and include it in my Linux builds. I used to move around files. So I am adding my final CMakeLists.txt and build instructions here. Please approve the Pull request, if you find it useful. I have a fork of the cpp-splines, with OpenCV plotting out the beautiful splines, you have created. Thanks for this awesome repository