cmake: add doxygen
cmake: add doxygen

j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt