[CMake] Removing unused variables from the cache

[Brad King]

Peter Soetens wrote:
> Quoting Brad King <brad.king at kitware.com>:
>>
>> If BUILD_X is on and SOME_OPTION is set by the user, then BUILD_X is
>> turned off, and then back on the user may expect SOME_OPTION to have
>> retained its value.  You can convert it to an INTERNAL cache entry so
>> that it does not show up in the GUI when it is not needed and then use
>> FORCE to change its type back later.  Here is an example macro to do
>> that for on/off sub-options:
>>
>> http://www.vtk.org/cgi-bin/viewcvs.cgi/CMake/vtkDependentOption.cmake?rev=1.5&view=markup
>>
> 
> Thank you very much for this link ! It is this kind of 'high level'
> macro's that I'm missing most in 'standard' cmake.

By coincidence I just yesterday discovered a need for this macro in
CMake itself.  I've added it to the modules as
CMakeDependentOption.cmake.  I'm not sure whether it will make the cut
for 2.4.4 but it will be in a release by 2.4.5.

-Brad