[CMake] Add --help-option command line option.

[Rolf Eike Beer]

> Hi,
>
> I have noticed that most cmake based projects document their options
> in their README file whereas the documentation is already included in
> cmake when you call the option() and set() commands. I hope this
> feature will help to remove this duplication and people will just
> write in their README file a "see cmake --help-options for available
> build options".
>
> Comments?

This sounds like a good idea to me. Don't forget about
CMAKE_DEPENDENT_OPTION ;)

Eike