xfce4-session's configure script checks for version 4.4..0 (note the double dots) of libxfcegui4-1.0. On OpenBSD, this causes configure to fail because OpenBSD's pkg-config does not accept such version numbers.
Fixed with revision 24775. 2007-01-28 Benedikt Meurer <benny@xfce.org> * configure.in.in: Fix broken version number of libxfcegui4 dependency. Bug #2824.