Require CMake for all builds

This PR grealy cleans the build code by requiring CMake to build Rugged under all platforms. We've found that the Makefile.embed route is not ideal, particularly because it builds Rugged without SSH and HTTPS support, which confuses people.

I've had to remove the cross-compilation code because a) I don't use it, b) it's buggy becauses it uses the builting Makefile. If somebody wants to attempt to fix it, please go ahead.