[CMake] externalproject sanity check

[Clinton Stimpson]

Hi,

So I have this project that is built using cmake, but as a subdirectory, I 
have source for another project that doesn't use cmake to build (it has its 
own visual studio plugins for building and must be an in-source build).

Is the CMake 2.8 feature of ExternalProject suitable for this setup?
We've tried using it, but it insists on specifying a way to download the 
source even though we've set SOURCE_DIR.  And it sometimes deletes the source 
files in that subdirectory, so it appears to have its own "clean" instead of 
letting me specify one.

If not, I guess I'll have to fall back to having a custom target.

Thanks,
Clint