Add ci jobs for clang builds and reuse checks

clang ever so often spots issues gcc misses (and the other way around) so let's build with clang as well. We disable -Wcast-align as this gives us false positives (this also helps when e.g. using rtags.

Also add a license check. We do s.th. similar in phosh but use the reuse tool in this case as it's more flexible (it wasn't around when the checks were added in phosh).