dunecontrol: `dunecontrol --configure-flags=${flags}` and `dunecontrol configure ${flags}` do not work with cmake options

@simon.praetorius noticed that the dunecontrol --configure-flags=${FLAGS} and dunecontrol configure ${FLAGS} commands don't pass ${FLAGS} to cmake.

dunecontrol could be changed to pass them on, but I noticed that there is still some code to convert autotools options to cmake options for handling CXXFLAGS=... as ${FLAGS}. This would break and users would have to specify -DCMAKE_CXX_COMPILER=... instead.

I believe we should still change dunecontrol as there is currently no way to pass cmake-specific options (-D...) to cmake via dunecontrol's command line arguments.