Fix distclean/maintainer-clean targets to remove top-level tmp_install dir.

The top-level makefile removes tmp_install in its "clean" target, but the
distclean and maintainer-clean targets overlooked that (and they don't
simply invoke clean, because that would result in an extra tree traversal).

While at it, let's just make sure that removing GNUmakefile itself is the
very last step of the recipe.