Given the license of some files is dubious, this allows MIT loving people to install them as files and not bundling them.
make test fails ATM, but that needs more love if that idea is accepted at all.
BTW, slightly related as idea: I think it would be nice to add a function Definition Downloader::setDownloadLocation() or similar that allows to have a custom download location. This would match nicely with Repository::addCustomSearchPath().
Note: The Ninja generator doesn't like this:
There must be a way to implement this feature and keep using add_custom_command...? Using execute_process() is usually a bad sign (tm). Note that these commands will be executed each CMake run(!)