Suppress the warning "Manually-specified variables were not used by the project"
Summary
The script dunecontrol passes some variables to cmake that are used only conditionally. If they are not used then CMake will raise a warning. This is annoying and irritating for the users who might think they have done something wrong but haven't. The easiest way to suppress this warning is to actually use the variable. Therefore, I have introduced another internal CMake variable DUNE_MAYBE_UNUSED_VARIABLES that contains the evaluated unused variables.
