In the latest commit (and the one right before it that looks like it was intended to be a release commit?), there is no configure script, which makes the install instructions in the README invalid.
Indeed, it should say "autogen.sh" instead of configure. Pushed an updated readme: https://git.xfce.org/panel-plugins/xfce4-clipman-plugin/commit/?id=4229359a104074ee55b79f3a4ab682e9c5beace5