doxygen_add_docs doesn't support specifying a doxyfile
According to the documentation (and my personal experience), CMake does not support passing a doxyfile to doxygen via the doxygen_add_docs()
command. It seems CMake is only willing to use a doxyfile it auto-generates.
If that is not actually the case, then this is a bug report about the documentation - as I was not able to figure out how to specify a doxyfile (nor how to override doxyfile entries; how relative paths within the doxyfile are interpreted etc.)