In CMakeLists.txt, FIND_MODE is not defined here. This was moved from
UI/CMakeLists.txt in commit cb4d2ff7. In
UI/CMakeLists.txt, FIND_MODE is REQUIRED if DISABLE_UI is false or
undefined and ENABLE_UI is true. Since the same booleans are required
for the if-else branch in CMakeLists.txt where we try to find Qt, we can
set find_package to REQUIRED here as well.