Visual Studio Express editions do not support solution folders,
so default behavior should be as if USE_FOLDERS global property
is OFF.

Also, allow folder names to be the same as target names: internally,
use a prefix to distinguish folder GUIDs from target GUIDs. Add
a target and folder with the same name in the ExternalProject
test to exercise this code.

For CMake itself, provide a new option CMAKE_USE_FOLDERS that
defaults to ON so that Visual Studio users get a nicely organized
CMake project. Express edition users will have to turn off the
CMAKE_USE_FOLDERS option in order to build CMake in the VS Express
IDE.