automagic dep on doxygen

[ZeroChaos-]

-DENABLE_DOXYGEN=OFF does not appear to actually work

cmake --no-warn-unused-cli -C /var/tmp/portage/net-wireless/gr-paint-9999/work/gr-paint-9999_build/gentoo_common_config.cmake -G Unix Makefiles -DCMAKE_INSTALL_PREFIX=/usr -DENABLE_DOXYGEN=OFF -DPYTHON_EXECUTABLE=/usr/bin/python2.7 -DCMAKE_BUILD_TYPE=Gentoo -DCMAKE_INSTALL_DO_STRIP=OFF -DCMAKE_USER_MAKE_RULES_OVERRIDE=/var/tmp/portage/net-wireless/gr-paint-9999/work/gr-paint-9999_build/gentoo_rules.cmake -DCMAKE_TOOLCHAIN_FILE=/var/tmp/portage/net-wireless/gr-paint-9999/work/gr-paint-9999_build/gentoo_toolchain.cmake /var/tmp/portage/net-wireless/gr-paint-9999/work/gr-paint-9999

-- Could NOT find Doxygen (missing: DOXYGEN_EXECUTABLE)

pretty sure it shouldn't be looking for doxygen if I have disabled it on the cli