Fix installation of empty directory if DESTDIR environment variable is set

This patch should fix https://github.com/orocos-toolchain/rtt/commit/db7879ab36d16ec062e828c2ce33ea59fe3a1e5e#commitcomment-5313342. Tested locally with git-bloom-release and dpkg-buildpackage as an unprivileged used without write permission to the real install directory.