[CMake] unset() command

[Brad King]

Philip Lowman wrote:
> So I submitted a patch to add a new unset() command to CMake.  The
> command will unset normal or cache variables (causing them to become
> undefined).  The command also supports unsetting environment variables,
> in case anyone has to do that.
> 
> The primary use case that drove me to implement this was this:
> 
> find_library(FOO_LIBRARY foo)
> if(MSVC OR SEARCH_FOO_DEBUG)
>     find_library(FOO_LIBRARY_DEBUG food)
> else()
>     unset(FOO_LIBRARY_DEBUG)
>     set(FOO_LIBRARY_DEBUG ${FOO_LIBRARY})
> endif()
> 
> http://public.kitware.com/Bug/view.php?id=7507

I think the best place for discussion on this is in the bug entry
itself.  I'll make comments there.

-Brad