[CMake] Can CMake prompt the user for input while it's running?

[Bill Hoffman]

> 
> Off course the cmake UI would authorize to unfilter all vars
> just as today there is the advanced/normal view.

You could just use the advanced/non-advanced to handle this. When you 
want to show a variable to the user make it non-advanced.  This would 
work with ccmake, cmake -i, and cmake-gui.  They all default to showing 
non-advanced variables by default.

-Bill