Fix CMake warning about minimum version when trying to find KF5

Fix CMake warning about minimum version when trying to find KF5

Disable even trying to build the GUI when we deal with a CMake older
than 2.8.12.

CMake Warning (dev) at /home/milian/projects/compiled/kf5/share/ECM/modules/ECMFindModuleHelpers.cmake:133 (message):
  Your project should require at least CMake 2.8.12 to use FindKF5.cmake
Call Stack (most recent call first):
  /home/milian/projects/compiled/kf5/share/ECM/find-modules/FindKF5.cmake:52 (ecm_find_package_version_check)
  CMakeLists.txt:35 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.