Added CMake Support

[darren-harton]

I added basic CMake support so that other projects may use CMake's externalproject_add and find_package to use this library. Tests are not being built, but that can be added if requested.

Testing:

i7-9750H, 6 cores, 12 threads
Ubuntu 20.04.5 LTS
gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
CMake 3.16.3

I compiled another project using your header files as dependencies. A sample project can be provided if requested.

[Mandrenkov]

Thanks @darren-harton, I really appreciate the contribution! I'm not really a CMake expert myself, but I have no issue with adopting it as the official way to build the library. 👍

Before merging, there are a few things which should be addressed. These are:

1. Running cmake . generates a bunch of build artifacts in the root project directory. Ideally, these should be generated inside a build folder to avoid polluting the filesystem.
2. The files generates by CMake are not captured in the .gitignore but they definitely should be! We should be able to copy over this one with some minor adjustments.
3. If we're going to delete the Makefile outright, we'll need a way to run the tests as well. Otherwise, they may as well be dead code, right?
4. The README instructions should need to updated for CMake. I would recommend replacing the current Makefile recipes with CMake invocations to keep things accessible.