Ensure all necessary files are included in the tarball produced by "make dist"

make dist should produce a self-contained tarball which can be used for building and installing BIND. Since e.g. multiple headers are currently not included in *_SOURCES Automake variables, this is currently not the case and it needs to be fixed so that we can use make dist instead of git archive (+ .gitattributes) for creating our source tarballs.