extra-cmake-modules prefers K_DOXYGEN instead of DOXYGEN_SHOULD_SKIP_THIS
extra-cmake-modules/modules/ECMQchDoxygen.config.in says "# K_DOXYGEN set to have preprocessor macros know that kapidox/doxygen processes them
DOXYGEN_SHOULD_SKIP_THIS is the deprecated variant (remove for KF6)
PREDEFINED = DOXYGEN_SHOULD_SKIP_THIS
K_DOXYGEN ".