Add CMakeLists to project

Yeah, I know that CMake is a bit controversial, however this shouldn't effect
the existing autotools build. CMake also has a few advantages over autotools:

- It will generate a project file for your IDE of choice
- It makes building software under Windows less painful

The second point in particular is my inspiration for this commit; I would like
to port audiofile to Windows and while the code is rather portable, the build
system is not.
Add config.h for Windows builds

In order to avoid conflicts with the config.h generated by autotools, this is
placed out of the way and instead included with CMake via include_directory.