Fix find doxygen
Fix find doxygen

It seems that DOXYGEN_DOT_FOUND is not set correctly if it is required
as package component, at least not on cmake 3.10 and 3.16. CMake 3.9
introduced an improved FindDoxygen module, so lets set this manually
for CMake 3.9 or newer.

cutelyst/cutelyst,buschmann23/cutelyst,buschmann23/cutelyst,cutelyst/cutelyst,cutelyst/cutelyst,buschmann23/cutelyst