Previously, Doxygen needed to be ran from the project's source root dir due to the relative paths in the config's `INPUT` tag. We now preprocess the relative paths by prefixing them with the absolute path of the project's source root dir. The HTML output remains unchanged. Fixes: https://git.lix.systems/lix-project/lix/issues/240 Change-Id: I85f099c22bfc5fdbf26be27c2db7dcbc8155c8b2