[CMake] ADD_SUBDIRECTORY

[George Neill]

Hi Alexander,

> I think it may be not that hard, it also makes some sense, but to me this
> would feel a bit like bloat.

Yeah possibly ... it certainly would have saved me a few minutes!
(not that it really matters, in the end I was able to make CMake do
what I needed without too much hassle).  If anyone else would find it
useful ... I will grab the source and make it happen.

Thanks much,
George.