XFCE uses the symbol `wnck_screen_get_workspaces', which is not available in libwnck 2.18.x. The installer (and, evidently, configure scripts) check only for >= 2.12.x. This results in a link error later in the build process. The build is successful with libwnck >= 2.24. I do not know if 2.20 or 2.22 works.
Already fixed in trunk (my bad), r29403.