Support for CMake

[rossb34]

Define a CMakeLists.txt file such that the hffix library can easily be included as an external project for applications and libraries that use CMake.

[jamesdbrock]

Hi @rossb34 , I'm open to this, but I don't use Cmake. Do you want to make a PR?

We would also want a .nix shell file which provides a CMake environment that demonstrates that the CMakeLists.txt file is working. I can do that part, if you don't use Nix.