[Cmake] Letting SOURCE_GROUP get a list of file names from a variable

[Brad King]

> This should already work.  All commands have ExpandListArguments invoked
> automatically for non-quoted arguments.  Don't forget about the bug you
> reported earlier.  Try this with an umodified 1.6.7:
>
> SET(var FILES alpha.cpp bravo.cpp)
> SOURCE_GROUP("Some Files" FILES . ${var})

Okay, I've just read the source code, and I see that the FILES signature
of the SOURCE_GROUP command was not documented.  This is because it was
never finished.  I've now totally re-written the implementation in the CVS
version of CMake.  It will be available in the next release.

-Brad