Make CMake work with disabled version check.

Before this commit setting DISABLE_CXX_VERSION_CHECK to true resulted in the CMake error:

CMake Error at cmake/modules/CheckCXXFeatures.cmake:110 (if): if given arguments:

"14" "GREATER"

Unknown arguments specified Call Stack (most recent call first): cmake/modules/CheckCXXFeatures.cmake:199 (dune_require_cxx_standard) cmake/modules/DuneMacros.cmake:690 (include) CMakeLists.txt:17 (dune_project)