[bugfix,2.6] Fix CMake with deactivated compiler version check

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)

IMHO this should be in 2.6 as people might need to get compilers working, we did not think about.

changed title from [bugfix,2.6] Fix deactivated version check to [bugfix,2.6] Fix CMake with deactivated compiler version check

changed the description

Damn will need to make this on master.

closed