[CMake] Copying DLLs to output directory

[Robert Dailey]

This actually used to be a very difficult problem to solve. However,
to debug in visual studio it's essential.

If I have DLLs located in third party directories OR from targets that
I depend on, those must all be copied to the directory of the
executable I'm debugging in order for those DLLs to be found and
loaded.

Using CMake 3.0.2, I hope this task is simpler, especially with the
introduction of a nice suite of generator expressions. Can anyone
recommend a good way to do this?