Support installing from cmake, or emit an error when it's tried

[cpu]

If folks miss the note that cmake is only required for the client/server example it might be tempting to try and use cmake to install rustls-ffi.

We're in bad shape for that case right now because it:

a) doesn't work!
b) doesn't emit any kind of error!

Making it actually work might require work towards #519 since we're presently "installing" the library to a fixed location in the example binary build dir as a pre-req for the client/server binaries.

Making it emit an error should be straight forward and an easy UX win.