Add install directives to CMake configuration

The CMake build can now also be used to install the generated binaries.

First of all, you have to set a global build prefix, especially if you
want the binaries to be deployed e.g. in a build root directory.

By default, CMake uses a relative build prefix of "/usr/local", so the
binaries would go to "/build/prefix/usr/local/bin". To change this to
"/build/prefix/usr/bin", set the CMAKE_INSTALL_PREFIX variable when
invoking CMake in your build subdirectory:

cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr ..

You can then set the global build root by using the DESTDIR variable of
make:

make -j<N> DESTDIR=/build/prefix install

This will compile everything and create files like
"/build/prefix/usr/bin/an-config" and so on.