[CMake] Modifying a variable's value without resetting the docstring

[David Doria]

> Well, if ${_var} is already a cache variable, you can retrieve it's HELPSTRING property to see what was set as the original doc string. But if it's not, then there won't be one, and you'll be adding an undocumented option...
>
>   http://cmake.org/cmake/help/v2.8.9/cmake.html#section_PropertiesonCacheEntries


Thanks Dave, that'll work:

        get_property(currentHelpString CACHE "${_var}" PROPERTY HELPSTRING)
        set(${_var} OFF CACHE BOOL ${currentHelpString} FORCE)